WTF are you doing where shell "speed" is the bottleneck? How does it happen that your shell starts "slow". This post has lots of speed comments. I don't get it. Are bash pipes slower than others? Do folk have a shell that takes longer than 250ms to start?
Not OP but fish's fuzzy autocomplete is snappy, especially on low-end hardware even when I mistype a lot, where zsh will often just freeze completely, for me at least.
I tried out many of the popular shells (bash, zsh, fish, etc) many years ago when my daily driver was an older thinkpad, and some of the startup times were painfully slow for me. It definitely drove me toward the more minimal shells like ksh and rc. On newer hardware it hasn't been noticeable, but it's real.