Every extension is a small program that loads with the browser and often runs code on each page you visit. Ten extensions means ten programs inspecting, rewriting or watching every page, and each one adds memory use and a little delay.
The slowdown is rarely dramatic from a single add-on, which is why it goes unnoticed. It appears gradually over months as extensions accumulate, until pages take noticeably longer to become interactive than they did on the same machine a year earlier.
Extensions can also break pages outright. Content blockers and privacy tools remove elements that a site depends on, producing blank sections or buttons that do nothing, and the site itself looks broken.
Private browsing usually runs with extensions disabled, which makes it the fastest test available. If a slow or broken page behaves perfectly in a private window, an extension is almost certainly responsible.