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 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.

Of course, with classic terminals you can redirect the compiler output, to avoid this problem, but I prefer to glance at the progress of the compilation, to notice early if there are problems.

Even in normal interactive use, there are enough cases when the difference between the instant rendering of ghostty or kitty and the slowness of the classic terminals is noticeable.

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.

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.