May be it's a know issue, but I was surprised to discover this behavior in my benchmarks.

Consider this two snippets of code:

for (var i = 0; i < buffer.length; i++) {
    ...
}

and

var length = buffer.length;
for (var i = 0; i < length; i++) {
    ...
}

under v2.5.0 their performance is indistinguishable, but under v3.0.0 or v3.1.0 the first version causes 1.5x slowdown of my whole benchmark.