Emacs can certainly be sluggish, but I'm not sure how much that's e.g. inherent to ELisp, or due to synchronous/single-threaded code, or choosing slow algorithms for certain tasks, etc.

For me, the best performance improvement has been handling long lines; e.g. Emacs used to become unusable if it was given a line of around 1MB. Since I run lots of shell-mode buffers, that would happen frustratingly-often. My workaround was to make my default shell a script that pipes `bash` through a pared-down, zero-allocation copy of GNU `fold`, to force a newline after hitting a certain length (crossing a lower threshold would break at the next whitespace; hitting an upper threshold would force a break immediately). That piping caused Bash to think it wasn't interactive, which required another work-around using Expect.

Thankfully the last few versions of Emacs have fixed long-line handling enough for me to get rid of my awful Rube-Goldberg shell!