I haven't seen anyone else mention Terminology yet. It uses an unconventional GUI framework (Enlightenment / EFL), but that aside, it's fast and has more or less all of the features you'd expect of a terminal:

https://github.com/borisfaure/terminology

Its "moment" as a new novel terminal was over a decade ago, but it still chugs on working just fine. Notably(?), gregkh uses it (or used to use it):

https://www.linuxfoundation.org/blog/blog/greg-kroah-hartman...