They do uxterm (which is a wrapper around xterm to put it into UTF-8 mode) here.
https://lwn.net/Articles/751763/
TL;DR: This quote sums it up:
"The first thing that struck me is that old programs like xterm and mlterm have the best response time, having worse case latency (2.4ms) better than the best case for all other terminals (10.6ms for st)."
I remember trying xterm, st and gnome terminal and gnome terminal was really bad. It felt comparable or maybe even worse than hyper. I also remember thinking "for such a less bloated terminal, st sure doesn't feel minimal". There was this level of latency present on st that xterm doesn't have.
xterm with tmux in my opinion is the ultimate combo. You get a lightning fast terminal and all of the features you could ever want (windows, panes, searching, etc.). With a bit of configuring, xterm can do all sorts of nice things like having clickable URLs, customized keybinds, font adjustment, etc.. Plus you can make it look good.