Well, sort of. The problem is it's difficult to implement a lot of extension functionality in efficient ways, and sometimes they make it too easy to do it in really really inefficient ways. Examples:
* LSP uses JSON. Even worse, VSCode uses UTF-16 internally (because Javascript does, more or less). But JSON uses UTF-8. So for correct operation VSCode converts the UTF-16 to UTF-8, and then your language server has to convert it back to UTF-16 to figure out the row/col positions (which are in UTF-16 units).
* LSP has an "easy mode" where the entire document is sent to the language server on every keypress!
* Some functions like `provideDocumentLinks()` operate on the entire document, so on every keypress (maybe it batches them I don't know), all linkifier extensions will run a load of regexes over the whole document.
There's definitely scope for a text editor with an extension API that encourages speed.
It is subjective and depends on your baseline. When you're used to fast editors ((neo)vim, sublime text back in the day, etc.), then VS Code is sluggish and bloated out of the box, and the idea of adding extensions that could make it worse is nightmare fuel.