I never moved to omz (or even zsh; I still rock bash) because I have 10+ years of dotfiles that cover all of my needs. I can see all of the cloud accounts I'm logged into, the status of my Git repositories when I'm cd'ed into them, the number of directories deep my stack is when I use pushd, and have lots and lots of custom functions that save me time. I had moving to zsh in my personal backlog for many years but have yet to come across a pressing reason why I should!
Anyway, yeah, all of this adds startup and command invocation time, but the value far outweighs the latency.
It's basically the same, except smarter about command history and auto-completion by default. I'm sure you could get Bash to act the same way with a bit of prodding.
I'm changing to GhosTTY as we speak (from Mac Terminal, because I'm tired of gemini-cli not getting my shift-return for newlines). Wasn't until that process that I learned that new Mac accounts are on a different shell (zsh) than I have (bash). Last time I switched shells was when Apple flipped from tcsh.
Now, I'm deep down the rabbit hole of standardizing all my shells/terminals/configs with Nix. Either a really good or really bad project to start on a Friday night.