I imagine you mean that servlets are the most performant web stack, not the most scalable web stack. Scalability != Performance.

Well, I'm sure it's easy to beat servlets with a load that requires only one thread with a simple enough computation model. Servlets (and any truly multithreaded solution) trades single-thread performance with the ability to scale with the number of cores. That's what I meant.

So I would think that given a heavy load on a many-core machine with interesting enough computations, nothing could beat servlets' performance.

My apologies - it's clear you were talking about scaling performance with additional cores, which is entirely the right usage of the term.