diff --git a/tools/isobuild/builder.js b/tools/isobuild/builder.js
index 707dbb3262..ea71eb8fc1 100644
--- a/tools/isobuild/builder.js
+++ b/tools/isobuild/builder.js
@@ -232,6 +232,39 @@ Previous builder: ${previousBuilder.outputPath}, this builder: ${outputPath}`
     return partsOut.join(files.pathSep);
   }
 
+  // Checks if a file with the same path and hash was written by
+  // the previous builder. If it was, it adds it to the cache and makes
+  // sure the parent directories exist and are part of the cache.
+  //
+  // Returns true if the file was already written
+  usePreviousWrite(relPath, hash, sanitize) {
+    relPath = this._normalizeFilePath(relPath, sanitize);
+
+    if (this.previousWrittenHashes[relPath] === hash) {
+      this._ensureDirectory(files.pathDirname(relPath));
+      this.writtenHashes[relPath] = hash;
+      this.usedAsFile[relPath] = true;
+      return true;
+    }
+
+    return false;
+  }
+
+  _normalizeFilePath(relPath, sanitize) {
+    // Ensure no trailing slash
+    if (relPath.slice(-1) === files.pathSep) {
+      relPath = relPath.slice(0, -1);
+    }
+
+    // In sanitize mode, ensure path does not contain segments like
+    // '..', does not contain forbidden characters, and is unique.
+    if (sanitize) {
+      relPath = this._sanitize(relPath);
+    }
+
+    return relPath;
+  }
+
   // Write either a buffer or the contents of a file to `relPath` (a
   // path to a file relative to the bundle root), creating it (and any
   // enclosing directories) if it doesn't exist yet. Exactly one of
@@ -254,16 +287,7 @@ Previous builder: ${previousBuilder.outputPath}, this builder: ${outputPath}`
   //
   // If `file` is used then it will be added to the builder's WatchSet.
   write(relPath, {data, file, hash, sanitize, executable, symlink}) {
-    // Ensure no trailing slash
-    if (relPath.slice(-1) === files.pathSep) {
-      relPath = relPath.slice(0, -1);
-    }
-
-    // In sanitize mode, ensure path does not contain segments like
-    // '..', does not contain forbidden characters, and is unique.
-    if (sanitize) {
-      relPath = this._sanitize(relPath);
-    }
+    relPath = this._normalizeFilePath(relPath, sanitize);
 
     let getData = null;
     if (data) {
@@ -291,7 +315,7 @@ Previous builder: ${previousBuilder.outputPath}, this builder: ${outputPath}`
 
       // Write is called multiple times for assets when they have multiple urls for the same file
       if (this.previousWrittenHashes[relPath] !== hash && this.writtenHashes[relPath] !== hash) {
-        
+
         // Builder is used to create build products, which should be read-only;
         // users shouldn't be manually editing automatically generated files and
         // expecting the results to "stick".
diff --git a/tools/isobuild/bundler.js b/tools/isobuild/bundler.js
index 3c097a5391..1adbfe88ae 100644
--- a/tools/isobuild/bundler.js
+++ b/tools/isobuild/bundler.js
@@ -176,6 +176,9 @@ import { PackageRegistry } from "../../packages/meteor/define-package.js";
 
 const SOURCE_URL_PREFIX = "meteor://\u{1f4bb}app";
 
+const MINIFY_PLAIN_FUNCTION = Buffer.from('function(', 'utf8');
+const MINIFY_RENAMED_FUNCTION = Buffer.from('function __minifyJs(', 'utf8');
+
 // files to ignore when bundling. node has no globs, so use regexps
 exports.ignoreFiles = [
     /~$/, /^\.#/,
@@ -546,6 +549,9 @@ class File {
     // disk).
     this.sourcePath = options.sourcePath;
 
+    // Allows not calculating sri when it isn't needed
+    this._skipSri = options.skipSri
+
     // info is just for help with debugging the tool; it isn't written to disk or
     // anything.
     this.info = options.info || '?';
@@ -619,7 +625,9 @@ class File {
         hashes.push(this._inputHash);
       }
 
-      hashes.push(this.sri());
+      if (!this._skipSri) {
+        hashes.push(this.sri());
+      }
 
       this._hash = watch.sha1(...hashes);
     }
@@ -628,7 +636,7 @@ class File {
   }
 
   sri() {
-    if (! this._sri) {
+    if (! this._sri && !this._skipSri) {
       this._sri = watch.sha512(this.contents());
     }
 
@@ -1160,6 +1168,7 @@ class Target {
           data: resource.data,
           cacheable: false,
           hash: resource.hash,
+          skipSri: !!resource.hash
         };
 
         const file = new File(fileOptions);
@@ -1326,13 +1335,10 @@ class Target {
         // expression, which some minifiers (e.g. UglifyJS) either fail to
         // parse or mistakenly eliminate as dead code. To avoid these
         // problems, we temporarily name the function __minifyJs.
-        file._contents = Buffer.from(
-          file.contents()
-            .toString("utf8")
-            .replace(/^\s*function\s*\(/,
-                     "function __minifyJs("),
-          "utf8"
-        );
+        file._contents = Buffer.concat([
+          MINIFY_RENAMED_FUNCTION,
+          file.contents().slice(MINIFY_PLAIN_FUNCTION.length)
+        ]);
 
         dynamicFiles.push(jsf);
 
@@ -1364,15 +1370,24 @@ class Target {
     function handle(source, dynamic) {
       source._minifiedFiles.forEach(file => {
         // Remove the function name __minifyJs that was added above.
-        file.data = file.data
-          .toString("utf8")
-          .replace(/^\s*function\s+__minifyJs\s*\(/,
-                   "function(");
+        if (typeof file.data === 'string') {
+          file.data = Buffer.from(
+            file.data
+              .replace(/^\s*function\s+__minifyJs\s*\(/,
+                       "function("),
+            "utf8"
+          );
+        } else if (dynamic) {
+          file.data = Buffer.concat([
+            MINIFY_PLAIN_FUNCTION,
+            file.data.slice(MINIFY_RENAMED_FUNCTION.length)
+          ]);
+        }
 
         const newFile = new File({
           info: 'minified js',
           arch,
-          data: Buffer.from(file.data, 'utf8'),
+          data: file.data,
           hash: inputHashesByJsFile.get(source),
         });
 
@@ -2744,6 +2759,10 @@ var writeFile = Profile("bundler writeFile", function (file, builder, options) {
   let data = file.contents();
   const hash = file.hash();
 
+  if (builder.usePreviousWrite(file.targetPath, hash)) {
+    return;
+  }
+
   if (options && options.sourceMapUrl) {
     data = addSourceMappingURL(data, options.sourceMapUrl);
   } else {