As you suggest, there are many ways to punish the bad citizens (slow or malicious extensions) without harming the good ones, which is what manifest v3 will do. You can solve with UX (e.g. showing that an extension is slowing down a page), or imposing strict guidelines on extensions for webrequest API times. Google is already quite strict on many aspects for extensions (e.g. obfuscated code was forbidden recently), so I don't see why they cannot be strict also with performance.