The majority of our slowdown vs. JSC on date-format-xparb is in its function String.leftPad: String.leftPad = function (val, size, ch) { var result = new String(val); if (ch == null) { ch = " "; } while (result.length < size) { result = ch + result; } return result; } I know JSC has optimizations for prepending to strings. A general prepend optimization may not be necessary. Other things that might work: - have ropes speed this up somehow - some kind of single-character prepend optimization - some kind of optimization specific to the pattern in this padding function JM wanted SS win: 15 ms

Looks like most of those milliseconds are spent in the call to new String(). Change var result = new String(val); to var result = "" + val; and you'll see it speed up by about 10ms. It looks like the Add stub is wasting a lot of time calling js_DefaultValue, so that could explain the rest of the slowdown.

We've caught V8 and JSC on xparb, so I think we're done.