I couldn’t quite put my finger on the why, but I was experiencing slower and slower startups of my terminal when using zsh (combined with the oh-my-zsh extension).
In my case, this was because of a rather long history file that gets loaded whenever you start a new terminal.
$ wc -l ~/.zsh_history 10005 /Users/mattias/.zsh_history
Turns out, loading over 10k lines worth of shell history whenever you launch a new shell is hard for a computer.
This was my fix:
$ cp ~/.zsh_history ~/.zsh_history.1 $ echo '' > ~/.zsh_history
I had to use echo because the shortcut that would normally work on Bash didn’t work here;
$ > ~/.zsh_history
Either way, that solved zsh from starting slowly for me.