rjsx is based on js2 which makes it unusually slow. js2 was a valiant attempt to make a major mode that uses elisp to actually parse the language. This gave it

rjsx is based on js2 which makes it unusually slow. js2 was a valiant attempt to make a major mode that uses elisp to actually parse the language. This gave it tremendous power compared to every non-Lisp major mode and most other JS editors at the time, but it has not aged especially well in modern JS support or performance. I’d try one of the LSP based approaches instead these days.