XTerm. I used to use rxvt-unicode, but it only supports 256 colors and gave me grief when I tried to get some emacs color theme working. There's only one thing I miss, which is that rxvt-unicode reflows lines when you resize the terminal, which xterm won't do. Oh and urxvtc starts very slightly faster, but no big deal.
I also looked at kitty, and I like that the author of that one tries to champion new features, like full keyboard support on par with X11 apps. But it takes noticeably longer to start and the latency also feels worse.
I have to ask. I launch new terminals with Super+Enter, I barely have time to release my key chord, and kitty is already opened. I understand "slower", but 100% slower than a couple tens milliseconds is still a couple tens of milliseconds. My WM/compositor popping up the window and shell probably take longer by themselves than the difference in launch times between those two.
YMMV depending on what you consider to be noticeable delay & latency, I guess?
Just tried this again. Kitty takes like maybe half a second to start on my machine (maybe yours is faster?). Not sure how to measure this. xterm starts almost instantly. I can type "Super+Enter ls" and it'll work. Doesn't work with kitty, the keystrokes just disappear. Is this actually important? Probably not, but it feels annoying. Like slow internet.
I might have imagined the typing latency, since it feels the same as xterm now. Maybe I'm remembering wrong. I was on the old Debian when I last tried this though, so something could have changed.