diff --git a/examples/list-length.rkt b/examples/list-length.rkt
index 81e7044..bb6899b 100644
--- a/examples/list-length.rkt
+++ b/examples/list-length.rkt
@@ -1,11 +1,13 @@
 #lang planet dyoo/whalesong
 
-(define (mylen x)
+(define (mylen x acc)
   (cond
     [(empty? x)
-     0]
+     acc]
     [else
-     (add1 (mylen (rest x)))]))
+     (mylen (rest x) (add1 acc))]))
 "computing length"
-(mylen (build-list 100000 (lambda (i) i)))
+(define v (build-list 1000000 (lambda (i) i)))
+(printf "Built list\n")
+(mylen v 0)
 "done computing length"
\ No newline at end of file
diff --git a/js-assembler/runtime-src/baselib-lists.js b/js-assembler/runtime-src/baselib-lists.js
index b003bf6..28a20bd 100644
--- a/js-assembler/runtime-src/baselib-lists.js
+++ b/js-assembler/runtime-src/baselib-lists.js
@@ -264,18 +264,26 @@
         var tortoise, hare;
         tortoise = hare = x;
         if (hare === EMPTY) { 
-            tortoise._isList = true;
             return true; 
         }
         while (true) {
             if (!(hare instanceof Cons)) { return false; }
             if (tortoise instanceof Cons) { 
-                if (tortoise._isList === true) { return true; }
                 tortoise = tortoise.rest; 
             }
             hare = hare.rest;
-            if (hare instanceof Cons) { hare = hare.rest; }
-            if (hare === EMPTY) { return true; }
+            if (hare instanceof Cons) { 
+                // optimization to get amortized linear time isList:
+                if (hare._isList) { tortoise._isList = true; return true; }
+                hare = hare.rest; 
+                // optimization to get amortized linear time isList:
+                if (hare instanceof Cons && hare._isList) { tortoise._isList = true; return true; }
+            }
+            if (hare === EMPTY) { 
+                // optimization to get amortized linear time isList:
+                tortoise._isList = true;
+                return true; 
+            }
             if (tortoise === hare) { return false; }
         }
     };
diff --git a/js-assembler/runtime-src/baselib-vectors.js b/js-assembler/runtime-src/baselib-vectors.js
index c9d600d..1bb72b6 100644
--- a/js-assembler/runtime-src/baselib-vectors.js
+++ b/js-assembler/runtime-src/baselib-vectors.js
@@ -15,10 +15,6 @@
             for (i = 0; i < n; i++) {
                 this.elts[i] = initialElements[i];
             }
-        } else {
-            for (i = 0; i < n; i++) {
-                this.elts[i] = undefined;
-            }
         }
         this.mutable = true;
     };
diff --git a/js-assembler/runtime-src/runtime.js b/js-assembler/runtime-src/runtime.js
index 655255a..1f50cd0 100644
--- a/js-assembler/runtime-src/runtime.js
+++ b/js-assembler/runtime-src/runtime.js
@@ -395,7 +395,8 @@
         var frame = this.c[this.c.length - 1];
         var marks = frame.marks;
         var i;
-        for (i = 0; i < marks.length; i++) {
+        var l = marks.length;
+        for (i = 0; i < l; i++) {
             if (key === marks[i][0]) {
                 marks[i][1] = value;
                 return;