It's definitely slower when doing any intensive background activities that Emacs would normally offload. But I've found VSCode has features readily available as one-click installs that are very difficult and convoluted to setup in Emacs. For some of those you end up either settling for less-ideal tools in Emacs, or because you're not an expert in the specifics of the tool being integrated, you end up with a much less optimized integration. And in either case you can actually end up with a net-worse performance in Emacs, even though the VSCode core is in a far less performant language than Emacs.