I've wondered this myself. I stuck with iTerm2 for a really long time but recently switched to Ghostty. Ghostty definitely seems to renders faster (I say "seems to" because I didn't benchmark or anything).
I don't actually use a multiplexer. I'm not sure I understand the use case when most terminal emulators supports tabs + windows (like iTerm2 does). Or you could just launch multiple windows and use your OS's window manager to "multiplex". Are the terminals connected in some way that I'm missing?
Regardless, I think Ghostty is a reminder that you can win in a space that's crowded. There's probably a lot of widely used software that would get dropped en masse immediately if someone had the guts to make a better alternative.
Typically, the benefit of the mux over just more windows or tabs is the ability to detach sessions from a term and have them continue to run. But yea, the “split a term window into multiple terms” doesn’t have quite as much value in a world of lots of tabs. That said, you can do it under program control on the remote host, so it does save remote login overhead, etc. But for local, less value.