[throwawayboise]

Emacs is still (AFAIK) entirely single-threaded. If you are performing some operation that blocks (e.g. a network call, a shell command such as "git clone") all other windows are blocked until that completes.

I would guess if you have something like FlyMake configured for on-the-fly syntax checking, you could also get a "laggy" feeling.

Just launching and using a vanilla emacs session feels quite snappy in my experience.

[d0mine]

you don't need multiple threads to run things concurrently (such as network I/O, communicating with subprocesses) e.g., I believe url-retrieve in emacs doesn't stop the world (because there is -synchroniously version too). async is a thing

[unhammer]

magit runs git clone and such asynchronously. There's usable concurrency support in Emacs for things like network calls and external processes even though UI and pure elisp stuff is still single-threaded