The relevant benchmark (on an Intel i7-4510U machine):

name                      old time/op  new time/op  delta
FormatFloat/Slowpath64-4  68.6µs ± 0%  44.1µs ± 2%  -35.71%  (p=0.000 n=13+15)

Change-Id: I67eb0e81ce74ed57752d0280059f91419f09e93b
Reviewed-on: https://go-review.googlesource.com/30099Reviewed-by: Robert Griesemer <gri@golang.org>