Is an assembly webserver more performant than webservers written in other languages? Are there any hard limits on how much you can squeeze when using a particular framework?
The language matters much less than the architecture of the web server. A server written in assembly using a fork-on-request model like ymawky is going to be much slower than a server written in C using an async event loop like nginx, because forking is very inefficient at scale. Plus a big bottleneck is the networking syscalls, rather than the code itself, so two servers with the same model written in Assembly and C would likely be roughly equal.
Assembly can provide the best raw performance because it's closer to the hardware. However, writing and maintaining a web server in Assembly takes much more time, and in many cases the performance gain over a well-written C program is small.