I benchmarked all the variations on Chrome, var foo = +new Date() was the fastest—not by a huge amount than the rest, but I remember there was one approach that was especially slow.

I did this while optimizing performance of a fast-paced game in Javascript (LineRage). Profiling showed that getting timestamps was responsible for significant portion of the processing time. I also ended up switching to a global clock approach.

Unfortunately I no longer have the original benchmark code/results. Would be happy to have someone replicate it.

And another form is:

  var I = Number(new Date);