That's for interactive use, startup time is slower for iTerm2 but i don't care about that because i basically never quit the terminal.
In both iTerm2 (when i used it) and Terminal, i have a colourscheme enabled and a custom font - both of which i'm assuming have potential to slow things down.
You don’t need to know any tmux to use it either (except for how to launch the right mode, which is easy to script).
If you spend a lot of time SSHed to other computers, I would highly recommend trying it out.
But then i need to give up nested tmux on remotes https://github.com/craigjperry2/dotfiles/blob/main/dotfiles/... - it's super convenient to have tmux nested remotely for organising work and locally for broadcasting or just convenient context switching. Even when i was an iTerm2 user, i felt vanilla tmux was just way more comfortable (copy paste buffer for example)
* Recognize something in the output that looks like a Jira ticket and add a link to that ticket
* Have your ssh passwords in one place and automatically enter them upon prompt
* Connect to many servers and type the same command into all of them
* Make your screen red when in superuser mode
* etc. etc. etc.
(Some years ago, I refused a company-issued ThinkPad Carbon and byod'ed a Mac because ThinkPad couldn't do iTerm2.)
I switched to WezTerm awhile ago for my main terminal emulator, a decision I've been happy with. But I keep a copy of iTerm running so I can pull down the Quake terminal. Main uses are running homebrew updates, and, perhaps ironically given the topic of this thread, sgpt.
More advanced search with regex support, more advanced paste (can do character encoding transformations, deal with special characters, etc), smarter and configurable text selection, autocomplete (mixed bag, TBH - I use zsh for that), more advanced snippets for repetitive commands, and triggers to notify you when things happen (long running commands finish, certain words pop up eg "error" or "compile done").
It has a basic integrated password manager that allows me to paste passwords I commonly use in the terminal with a keycombo.
It can more tightly integrate with the shell/program. You can select a point with the mouse in vim or the shell and the text cursor will go there, for example.
Some of these may have since poked their way into the built in terminal, but these are some of the main reasons I use iterm. If you spend a lot of time in the terminal, you can enhance your productivity.
PS It can even record the keystrokes and play back.