How does a non-performant terminal look? I don't think I've even seen one.

Serious question.

Print a massive log file in kitty/ghostty, and another one with you default terminal, and you'll see a massive difference in how fast the log is printed out. Well to be honest it's just comfort, I can't claim it makes a difference in actual productivity, for me the real benefits of kitty are just the terminal multiplexer-ish capabilities (panes, sessions), other rendering niceties like adjusting the font, or having smooth scroll, sensible default shortcuts. But the actual "performance" e.g. input lag or rendering lag, I don't think it's that important.

I have previously seen programs run much more slowly when launched from the built-in terminal in a JetBrains IDE because log output was blocked.

iterm2. When there's heavy logging, I've seen it lag. Doesn't feel as snappy as Kitty.