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

Serious question.

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.

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.

A snappy terminal doesn't prevent you from seeing half-filled screens. Flicker can occur when you write line-by-line. That happened in a recent TUI application I wrote. Using synchronized update mode solved the problem.

I frequently compile big software projects, using fast multi-core CPUs, like AMD 9950X, with which the compilation itself is really fast.

Such compilations produce a ton of text messages. With any classic terminal, the text rendering in the terminal window becomes a bottleneck, slowing down the compilation very noticeably.

With modern terminals that use GPU text rendering, like ghostty and kitty, compilations are done much faster.

To see the difference between a slow terminal emulator, like you probably use, and a fast terminal emulator, you can do a "cat" from a text file of a few megabytes, and you will see the speed difference.

Yeah from what I've seen Ghostty is slightly more performant but they are both very performant.