diff --git a/tools/isobuild/builder.js b/tools/isobuild/builder.js
index 04a23919ec..797dca477c 100644
--- a/tools/isobuild/builder.js
+++ b/tools/isobuild/builder.js
@@ -1,7 +1,7 @@
 import assert from "assert";
 import {WatchSet, readAndWatchFile, sha1} from '../fs/watch';
 import files, {
-  symlinkWithOverwrite,
+  symlinkWithOverwrite, realpath,
 } from '../fs/files';
 import NpmDiscards from './npm-discards.js';
 import {Profile} from '../tool-env/profile';
@@ -540,7 +540,7 @@ Previous builder: ${previousBuilder.outputPath}, this builder: ${outputPath}`
       // as well as node_modules/meteor and the parent directories of any
       // scoped npm packages.
       this._ensureAllNonPackageDirectories(
-        files.realpath(options.from),
+        realpath(options.from),
         options.to
       );
     }
@@ -637,7 +637,7 @@ Previous builder: ${previousBuilder.outputPath}, this builder: ${outputPath}`
       });
     }
 
-    const rootDir = files.realpath(from);
+    const rootDir = realpath(from);
 
     const walk = (absFrom, relTo) => {
       if (symlink && ! (relTo in this.usedAsFile)) {
@@ -661,7 +661,7 @@ Previous builder: ${previousBuilder.outputPath}, this builder: ${outputPath}`
           return;
         }
 
-        // Returns files.realpath(thisAbsFrom), iff it is external to
+        // Returns files.realpath(thisAbsFrom), if it is external to
         // rootDir, using caching because this function might be called
         // more than once.
         let cachedExternalPath;
@@ -671,7 +671,7 @@ Previous builder: ${previousBuilder.outputPath}, this builder: ${outputPath}`
           }
 
           try {
-            var real = files.realpath(thisAbsFrom);
+            var real = realpath(thisAbsFrom);
           } catch (e) {
             if (e.code !== "ENOENT" &&
                 e.code !== "ELOOP") {
diff --git a/tools/isobuild/bundler.js b/tools/isobuild/bundler.js
index cee56599cd..de60cc3aca 100644
--- a/tools/isobuild/bundler.js
+++ b/tools/isobuild/bundler.js
@@ -173,6 +173,7 @@ import { loadIsopackage } from '../tool-env/isopackets.js';
 import { CORDOVA_PLATFORM_VERSIONS } from '../cordova';
 import { gzipSync } from "zlib";
 import { PackageRegistry } from "../../packages/meteor/define-package.js";
+import { optimisticLStatOrNull } from '../fs/optimistic';
 
 const SOURCE_URL_PREFIX = "meteor://\u{1f4bb}app";
 
@@ -486,12 +487,11 @@ export class NodeModulesDirectory {
           return true;
         }
 
-        const real = files.realpathOrNull(path);
-        if (typeof real === "string" &&
-            real !== path) {
+        const fileStatus = optimisticLStatOrNull(path);
+        if (fileStatus && fileStatus.isSymbolicLink()) {
           // If node_modules/.bin/command is a symlink, determine the
           // answer by calling isWithinProdPackage(real).
-          return isWithinProdPackage(real);
+          return isWithinProdPackage(files.realpathOrNull(path));
         }
 
         // If node_modules/.bin/command is not a symlink, then it's hard
diff --git a/tools/isobuild/package-source.js b/tools/isobuild/package-source.js
index aa2ccdfbd7..253c5fda18 100644
--- a/tools/isobuild/package-source.js
+++ b/tools/isobuild/package-source.js
@@ -216,14 +216,44 @@ var getExcerptFromReadme = function (text) {
 class SymlinkLoopChecker {
   constructor(sourceRoot) {
     this.sourceRoot = sourceRoot;
+    this._realSourceRoot = files.realpath(sourceRoot);
     this._seenPaths = {};
+    this._cache = new Map();
   }
 
+  // Avoids running realpath unless necessary
+  // since it is relatively slow on windows
+  _realpath = Profile('_realpath', function (relDir) {
+    const absPath = files.pathJoin(this._realSourceRoot, relDir);
+
+    if (files.lstat(absPath).isSymbolicLink()) {
+      const result = files.realpath(absPath);
+      this._cache.set(relDir, result);
+
+      return result;
+  }
+
+    let result;
+    const parentDir = files.pathDirname(relDir);
+    const parentEntry = this._cache.get(parentDir);
+    if (parentDir === '.') {
+      result = absPath;
+    } else if (parentEntry) {
+      result = files.pathJoin(parentEntry, files.pathBasename(relDir));
+    } else {
+      // The parent dir was never checked, which prevents us from
+      // skipping realpath
+      result = files.realpath(absPath);
+    }
+
+    this._cache.set(relDir, result);
+    return result;
+  })
+
   check(relDir, quietly = true) {
-    const absPath = files.pathJoin(this.sourceRoot, relDir);
 
     try {
-      var realPath = files.realpath(absPath);
+      var realPath = this._realpath(relDir);
     } catch (e) {
       if (!e || e.code !== 'ELOOP') {
         throw e;