diff --git a/AUTHORS b/AUTHORS
index 213b805baa8e2..75fdda50b4ec7 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -606,3 +606,4 @@ a license to everyone to use it as detailed in LICENSE.)
 * Abdelrahman Teima <abdelrahmanteima@gmail.com>
 * Jaap Aarts <jaap.aarts1@gmail.com>
 * Dimokritos Kolitsos <dkolitsos@protonmail.com>
+* Bas van Dijk <bas@van.dijk.ch>
diff --git a/src/lib/libdylink.js b/src/lib/libdylink.js
index 3d1c63d0362cd..45a27c5834c44 100644
--- a/src/lib/libdylink.js
+++ b/src/lib/libdylink.js
@@ -1307,27 +1307,22 @@ var LibraryDylink = {
 #if DYLINK_DEBUG
     dbg('dlsym_js:', symbol);
 #endif
-    var result;
-    var newSymIndex;
-
     var lib = LDSO.loadedLibsByHandle[handle];
 #if ASSERTIONS
     assert(lib, `Tried to dlsym() from an unopened handle: ${handle}`);
 #endif
-    newSymIndex = Object.keys(lib.exports).indexOf(symbol);
-    if (newSymIndex == -1 || lib.exports[symbol].stub) {
+    if (!hasOwn(lib.exports, symbol) || lib.exports[symbol].stub) {
       dlSetError(`Tried to lookup unknown symbol "${symbol}" in dynamic lib: ${lib.name}`)
       return 0;
     }
+    var exportName = symbol;
 #if !WASM_BIGINT
     var origSym = 'orig$' + symbol;
-    result = lib.exports[origSym];
-    if (result) {
-      newSymIndex = Object.keys(lib.exports).indexOf(origSym);
+    if (lib.exports[origSym]) {
+      exportName = origSym;
     }
-    else
 #endif
-    result = lib.exports[symbol];
+    var result = lib.exports[exportName];
 
     if (typeof result === 'function') {
 #if DYLINK_DEBUG
@@ -1355,6 +1350,10 @@ var LibraryDylink = {
 #if DYLINK_DEBUG
         dbg('adding symbol to table:',  symbol);
 #endif
+        // Other threads replay this insertion by the symbol's enumeration index
+        // (`_dlsym_catchup_js`).  Using `indexOf` here is O(N),
+        // so only do so when a slot was actually allocated.
+        var newSymIndex = Object.keys(lib.exports).indexOf(exportName);
         {{{ makeSetValue('symbolIndex', 0, 'newSymIndex', '*') }}};
       }
     }
diff --git a/test/codesize/test_codesize_hello_dylink_all.json b/test/codesize/test_codesize_hello_dylink_all.json
index 0c03a480d8bfd..6826cfa3a0965 100644
--- a/test/codesize/test_codesize_hello_dylink_all.json
+++ b/test/codesize/test_codesize_hello_dylink_all.json
@@ -1,7 +1,7 @@
 {
-  "a.out.js": 272930,
+  "a.out.js": 272950,
   "a.out.nodebug.wasm": 589512,
-  "total": 862442,
+  "total": 862462,
   "sent": [
     "IMG_Init",
     "IMG_Load",