diff --git a/History.md b/History.md
index b360b74088..69ebe569e1 100644
--- a/History.md
+++ b/History.md
@@ -81,6 +81,11 @@ N/A
     * Updated V8 to [release v7.8](https://v8.dev/blog/v8-release-78) which includes improvements in performance, for example, object destructuring now is as fast as the equivalent variable assignment.
   * [12.15.0](https://nodejs.org/en/blog/release/v12.15.0/)
 
+* `cursor.observeChanges` now accepts a second options argument.
+  If your observer functions do not mutate the passed arguments, you can specify
+  `{ nonMutatingCallbacks: true }`, which improves performance by reducing
+  the amount of data copies.
+
 ## v1.9, 2020-01-09
 
 ### Breaking changes
diff --git a/packages/accounts-base/accounts_server.js b/packages/accounts-base/accounts_server.js
index fc0d35a89d..afed16aba8 100644
--- a/packages/accounts-base/accounts_server.js
+++ b/packages/accounts-base/accounts_server.js
@@ -895,7 +895,7 @@ export class AccountsServer extends AccountsCommon {
           // The onClose callback for the connection takes care of
           // cleaning up the observe handle and any other state we have
           // lying around.
-        });
+        }, { nonMutatingCallbacks: true });
 
         // If the user ran another login or logout command we were waiting for the
         // defer or added to fire (ie, another call to _setLoginToken occurred),
diff --git a/packages/minimongo/local_collection.js b/packages/minimongo/local_collection.js
index 5ea83843e0..e767137089 100644
--- a/packages/minimongo/local_collection.js
+++ b/packages/minimongo/local_collection.js
@@ -669,17 +669,18 @@ LocalCollection._CachingChangeObserver = class _CachingChangeObserver {
       this.docs = new OrderedDict(MongoID.idStringify);
       this.applyChange = {
         addedBefore: (id, fields, before) => {
-          const doc = EJSON.clone(fields);
+          // Take a shallow copy since the top-level properties can be changed
+          const doc = { ...fields };
 
           doc._id = id;
 
           if (callbacks.addedBefore) {
-            callbacks.addedBefore.call(this, id, fields, before);
+            callbacks.addedBefore.call(this, id, EJSON.clone(fields), before);
           }
 
           // This line triggers if we provide added with movedBefore.
           if (callbacks.added) {
-            callbacks.added.call(this, id, fields);
+            callbacks.added.call(this, id, EJSON.clone(fields));
           }
 
           // XXX could `before` be a falsy ID?  Technically
@@ -701,10 +702,11 @@ LocalCollection._CachingChangeObserver = class _CachingChangeObserver {
       this.docs = new LocalCollection._IdMap;
       this.applyChange = {
         added: (id, fields) => {
-          const doc = EJSON.clone(fields);
+          // Take a shallow copy since the top-level properties can be changed
+          const doc = { ...fields };
 
           if (callbacks.added) {
-            callbacks.added.call(this, id, fields);
+            callbacks.added.call(this, id, EJSON.clone(fields));
           }
 
           doc._id = id;
@@ -1330,7 +1332,12 @@ LocalCollection._observeFromObserveChanges = (cursor, observeCallbacks) => {
     callbacks: observeChangesCallbacks
   });
 
-  const handle = cursor.observeChanges(changeObserver.applyChange);
+  // CachingChangeObserver clones all received input on its callbacks
+  // So we can mark it as safe to reduce the ejson clones.
+  // This is tested by the `mongo-livedata - (extended) scribbling` tests
+  changeObserver.applyChange._fromObserve = true;
+  const handle = cursor.observeChanges(changeObserver.applyChange,
+    { nonMutatingCallbacks: true });
 
   suppressed = false;
 
diff --git a/packages/minimongo/package.js b/packages/minimongo/package.js
index 047682d2a7..51b5fbc057 100644
--- a/packages/minimongo/package.js
+++ b/packages/minimongo/package.js
@@ -1,6 +1,6 @@
 Package.describe({
   summary: "Meteor's client-side datastore: a port of MongoDB to Javascript",
-  version: '1.4.5'
+  version: '1.5.0'
 });
 
 Package.onUse(api => {
diff --git a/packages/mongo/collection.js b/packages/mongo/collection.js
index a610481ba6..30d6c82da7 100644
--- a/packages/mongo/collection.js
+++ b/packages/mongo/collection.js
@@ -372,7 +372,10 @@ Object.assign(Mongo.Collection, {
       removed: function (id) {
         sub.removed(collection, id);
       }
-    });
+    },
+    // Publications don't mutate the documents
+    // This is tested by the `livedata - publish callbacks clone` test
+    { nonMutatingCallbacks: true });
 
     // We don't call sub.ready() here: it gets called in livedata_server, after
     // possibly calling _publishCursor on multiple returned cursors.
diff --git a/packages/mongo/mongo_driver.js b/packages/mongo/mongo_driver.js
index 4be68220e6..40a36b42bf 100644
--- a/packages/mongo/mongo_driver.js
+++ b/packages/mongo/mongo_driver.js
@@ -918,7 +918,7 @@ Cursor.prototype.observe = function (callbacks) {
   return LocalCollection._observeFromObserveChanges(self, callbacks);
 };
 
-Cursor.prototype.observeChanges = function (callbacks) {
+Cursor.prototype.observeChanges = function (callbacks, options = {}) {
   var self = this;
   var methods = [
     'addedAt',
@@ -931,8 +931,8 @@ Cursor.prototype.observeChanges = function (callbacks) {
   ];
   var ordered = LocalCollection._observeChangesCallbacksAreOrdered(callbacks);
 
-  // XXX: Can we find out if callbacks are from observe?
-  var exceptionName = ' observe/observeChanges callback';
+  let exceptionName = callbacks._fromObserve ? 'observe' : 'observeChanges';
+  exceptionName += ' callback';
   methods.forEach(function (method) {
     if (callbacks[method] && typeof callbacks[method] == "function") {
       callbacks[method] = Meteor.bindEnvironment(callbacks[method], method + exceptionName);
@@ -940,7 +940,7 @@ Cursor.prototype.observeChanges = function (callbacks) {
   });
 
   return self._mongo._observeChanges(
-    self._cursorDescription, ordered, callbacks);
+    self._cursorDescription, ordered, callbacks, options.nonMutatingCallbacks);
 };
 
 MongoConnection.prototype._createSynchronousCursor = function(
@@ -1241,7 +1241,7 @@ MongoConnection.prototype.tail = function (cursorDescription, docCallback, timeo
 };
 
 MongoConnection.prototype._observeChanges = function (
-    cursorDescription, ordered, callbacks) {
+    cursorDescription, ordered, callbacks, nonMutatingCallbacks) {
   var self = this;
 
   if (cursorDescription.options.tailable) {
@@ -1282,7 +1282,10 @@ MongoConnection.prototype._observeChanges = function (
     }
   });
 
-  var observeHandle = new ObserveHandle(multiplexer, callbacks);
+  var observeHandle = new ObserveHandle(multiplexer,
+    callbacks,
+    nonMutatingCallbacks,
+  );
 
   if (firstHandle) {
     var matcher, sorter;
diff --git a/packages/mongo/mongo_livedata_tests.js b/packages/mongo/mongo_livedata_tests.js
index 30ea3c10e1..8f77d1c822 100644
--- a/packages/mongo/mongo_livedata_tests.js
+++ b/packages/mongo/mongo_livedata_tests.js
@@ -560,6 +560,80 @@ Tinytest.addAsync("mongo-livedata - scribbling, " + idGeneration, function (test
   onComplete();
 });
 
+if (Meteor.isServer) {
+  Tinytest.addAsync("mongo-livedata - extended scribbling, " + idGeneration, function (test, onComplete) {
+    function error() {
+      throw new Meteor.Error('unsafe object mutation');
+    }
+    
+    const denyModifications = {
+      get(target, key) {
+        const type = Object.prototype.toString.call(target[key]);
+        if (type === '[object Object]' || type === '[object Array]') {
+          return freeze(target[key]);
+        } else {
+          return target[key];
+        }
+      },
+      set: error,
+      deleteProperty: error,
+      defineProperty: error,
+    };
+    
+    // Object.freeze only throws in silent mode
+    // So we make our own version that always throws.
+    function freeze(obj) {
+      return new Proxy(obj, denyModifications);
+    }
+
+    const origApplyCallback = ObserveMultiplexer.prototype._applyCallback;
+    ObserveMultiplexer.prototype._applyCallback = function(callback, args) {
+      // Make sure that if anything touches the original object, this will throw
+      return origApplyCallback.call(this, callback, freeze(args));
+    }
+    
+    const run = test.runId();
+    const coll = new Mongo.Collection(`livedata_test_scribble_collection_${run}`, collectionOptions);
+    const expectMutatable = (o) => {
+      try {
+        o.a[0].c = 3;
+      } catch (error) {
+        test.fail();
+      }
+    }
+    const expectNotMutatable = (o) => {
+      try {
+        o.a[0].c = 3;
+        test.fail();
+      } catch (error) {}
+    }
+    const handle = coll.find({run}).observe({
+      addedAt: expectMutatable,
+      changedAt: function(id, o) {
+        expectMutatable(o);
+      }
+    });
+
+    const handle2 = coll.find({run}).observeChanges({
+      added: expectNotMutatable,
+      changed: function(id, o) {
+        expectNotMutatable(o);
+      }
+    }, { nonMutatingCallbacks: true });
+
+    runInFence(function () {
+      coll.insert({run, a: [ {c: 1} ]});
+      coll.update({run}, { $set: { 'a.0.c': 2 } });
+    });
+ 
+    handle.stop();
+    handle2.stop();
+
+    ObserveMultiplexer.prototype._applyCallback = origApplyCallback;
+    onComplete();
+  });
+}
+
 Tinytest.addAsync("mongo-livedata - stop handle in callback, " + idGeneration, function (test, onComplete) {
   var run = Random.id();
   var coll;
diff --git a/packages/mongo/observe_multiplex.js b/packages/mongo/observe_multiplex.js
index aa766bc07f..6e8f9349f6 100644
--- a/packages/mongo/observe_multiplex.js
+++ b/packages/mongo/observe_multiplex.js
@@ -155,11 +155,7 @@ _.extend(ObserveMultiplexer.prototype, {
         return;
 
       // First, apply the change to the cache.
-      // XXX We could make applyChange callbacks promise not to hang on to any
-      // state from their arguments (assuming that their supplied callbacks
-      // don't) and skip this clone. Currently 'changed' hangs on to state
-      // though.
-      self._cache.applyChange[callbackName].apply(null, EJSON.clone(args));
+      self._cache.applyChange[callbackName].apply(null, args);
 
       // If we haven't finished the initial adds, then we should only be getting
       // adds.
@@ -179,7 +175,8 @@ _.extend(ObserveMultiplexer.prototype, {
           return;
         var callback = handle['_' + callbackName];
         // clone arguments so that callbacks can mutate their arguments
-        callback && callback.apply(null, EJSON.clone(args));
+        callback && callback.apply(null,
+          handle.nonMutatingCallbacks ? args : EJSON.clone(args));
       });
     });
   },
@@ -199,8 +196,8 @@ _.extend(ObserveMultiplexer.prototype, {
     self._cache.docs.forEach(function (doc, id) {
       if (!_.has(self._handles, handle._id))
         throw Error("handle got removed before sending initial adds!");
-      var fields = EJSON.clone(doc);
-      delete fields._id;
+      const { _id, ...fields } = handle.nonMutatingCallbacks ? doc
+        : EJSON.clone(doc);
       if (self._ordered)
         add(id, fields, null); // we're going in order, so add at end
       else
@@ -211,7 +208,9 @@ _.extend(ObserveMultiplexer.prototype, {
 
 
 var nextObserveHandleId = 1;
-ObserveHandle = function (multiplexer, callbacks) {
+
+// When the callbacks do not mutate the arguments, we can skip a lot of data clones
+ObserveHandle = function (multiplexer, callbacks, nonMutatingCallbacks = false) {
   var self = this;
   // The end user is only supposed to call stop().  The other fields are
   // accessible to the multiplexer, though.
@@ -231,6 +230,7 @@ ObserveHandle = function (multiplexer, callbacks) {
   });
   self._stopped = false;
   self._id = nextObserveHandleId++;
+  self.nonMutatingCallbacks = nonMutatingCallbacks;
 };
 ObserveHandle.prototype.stop = function () {
   var self = this;
diff --git a/packages/mongo/package.js b/packages/mongo/package.js
index 9df74a8ba4..e0b6852bba 100644
--- a/packages/mongo/package.js
+++ b/packages/mongo/package.js
@@ -71,6 +71,7 @@ Package.onUse(function (api) {
   api.export('MongoInternals', 'server');
 
   api.export("Mongo");
+  api.export('ObserveMultiplexer', 'server', {testOnly: true});
 
   api.addFiles(['mongo_driver.js', 'oplog_tailing.js',
                  'observe_multiplex.js', 'doc_fetcher.js',