I think I have like, 50 extensions though not all of them are active at the same time (that's how vscode works I think) and I don't really see the lag. I have a high refresh rate screen, I'm pretty used to differentiating between laggy and non laggy graphics, etc and still nothing stands out. Maybe some extensions are resource hogs but I think vscode provides stats about extension resource usage and how much an extension slows down the editor so people can easily check what is causing their slowdowns.