--- a/services/sync/modules/util.js
+++ b/services/sync/modules/util.js
@@ -547,17 +547,18 @@ this.Utils = {
 
   /**
    * Create an array like the first but without elements of the second. Reuse
    * arrays if possible.
    */
   arraySub: function arraySub(minuend, subtrahend) {
     if (!minuend.length || !subtrahend.length)
       return minuend;
-    return minuend.filter(i => subtrahend.indexOf(i) == -1);
+    let setSubtrahend = new Set(subtrahend);
+    return minuend.filter(i => !setSubtrahend.has(i));
   },
 
   /**
    * Build the union of two arrays. Reuse arrays if possible.
    */
   arrayUnion: function arrayUnion(foo, bar) {
     if (!foo.length)
       return bar;