It isn't. I still use it once in a while on an old netbook with a couple of gigs of memory, and it flies for any editing task. In comparison, no IDE can even hope to run on that unit.

HOWEVER, a heavily and naively reconfigured emacs, possibly with uncompiled elisp code and copy-pasted invocations that wait for network responses could take ages to open.

If you are finding emacs to be slow, try it vanilla first ("--no-init-file") before deciding it's the program and not your configuration.