I would say that it’s extremely unlikely.
Websites in general are never limited by raw code execution, they are mostly limited by IO. Be that disk IO as files are read and written, database IO as you need to execute complex queries to gather all the data to build the user timeline, and network IO to transfer data to and from the user. For decentralized social media like Kbin or Lemmy its even more IO limited as each instance needs to go back and forth to other instances to keep up-to-date data.
Websites usually benefit much more from caching and in-memory databases to keep frequently used data in fast storage.
This is why simple, high level, object oriented, garbage collected languages have become so common. All the CPU performance penalties they incur don’t actually affect the website performance.
What does this even mean. Chromium or Webkit are not “native” to an OS. OSs don’t magically include browser engines, its not a critical component of an OS either.
Most OSs do come with browsers preinstalled, but they are programs just like any other. You can remove Safari from macOS (albeit its pretty hard because root is read only and signed), you can remove Edge from Windows. In my desktop with Windows 10 the only browser I have is Firefox (not even Edge), does that make Gecko the “native” browser engine?
If anything, the native browser engine for Windows would be MSHTML from Internet Explorer.