Yep, VSCode is slow because it is built on Electron which is just a another browser.

VSCode is only fast if you're comparing it to Atom.

extensions tend to be the slow part in my experience. after a couple heavy extensions on an already struggling work laptop I'll frequently outpace it's input handling and have to wait for it to catch up

Depends on what you're used before I guess. I came from sublime text which was written in C++ and was blazing fast. You could throw any size file at it, I still use it when I need to edit a large file. I can notice the input lag in vs code even in small files with a vanilla setup. After adding plugins the lag can become even more noticable and in certain use cases it straight up slows you down. It's not so slow that it's unusable, but it's noticably slower, and leaves me desiring more speed. But speed alone isn't enough, it needs really good plugins which is why I traded speedy sublime text for vs code in the first place.