Gentoo stable requires bash for baselayout-1
Gentoo unstable uses
I know - I wrote OpenRC and maintained baselayout-1 for a long time
For pure startup, busybox sh is the fastest, followed by dash followed by bash.
busybox is fastest purely because it doesn't fork as much due to the apps builtin. Plus the apps in question generally have much shorter code-paths due to many GNU extensions being stripped.
I didn't test zsh or pdksh for speed.