Enter about:processes in the address bar.
There's a speedometer icon next to the extensions row which will monitor resource usage for 5 seconds. There's a wealth of information.
this post was submitted on 28 Aug 2023
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cool!!
The answer to your actual question is really complicated though, and very difficult to know for any given extension.
Take ublock for example, it needs to examine every element on every page, which is generally "bad", but it's going to remove resource hungry ads, which is "good". The good part won't show up in about:processes.
That said, browsers (supported by modern cpus) are so insanely good at examining elements that you just won't notice the performance hit of a few dozen ms.
Problems only really arise when you encounter bugs.
Everything a computer does uses resources so every extension will have some performance impact, though you usually won't notice the difference from any one extension. As a general rule of thumb, the more an extension does the more resources it needs and so the bigger the impact. Some extensions free up additional resources (eg by blocking ads or trackers) which may also result in a net performance gain dependant on the page you're viewing.
A large extension on disk doesn't necessarily use more memory and CPU time than a small one, but it is more likely to. The only reliable way to tell which extensions are resource hogs is by benchmarking them.
Disabling extensions not in use is a good idea, though personally I'd uninstall them instead.