Lemmy's frontend uses the React-like InfernoJS library that claims to be faster than React however I really doubt it's frontend rendering that is the bottleneck here.
The frontend reacts to a click quite fast but there's still a spinner for a lot of actions. The API goes over a websocket, request-response delay seems to be about 300-400ms for me so it's not super snappy, but not jira hell either.
WOW! It was incredibly responsive. What the heck? That was so weird, there was no delay in clicking.
I didn't even realize this was a thing until it was so fast that I came here to talk about it.
Not going to speculate, just impressed.