What do you mean by slow? Like, slow at doing what? I can't think of a specific feature that is slower on vscode. Emacs is actually rather slow...
It depends on so many different things. Aside from platform specifics (sure, it can be painfully slow on Windows), Emacs might feel slow for various reasons, but it doesn't have to be. Built-in Profiling tools are godsend and if used properly Emacs can be quite impressively fast. My Emacs starts quickly (in less than a second), and it is very pleasantly snappy.
I am not sure how the speed of both compares, if one properly sets up Emacs on the server as well, to make it a fair comparison.