I would first blame the programmers, the design and lack of specialty offloading before blaming any programming language. Well designed web calls scale nearly linearly with usage and usually poor design or programming is the source of slowness. You can always trade language complexity for speed but assuming it is the cause of all perceived slowness is a poor man's view.

It is the same story every time again, first it was java, which has so many large scale projects most people won't even know it's running things they use, now it's apparently python who is to blame for all slowness on the web. When the next JIT or scripted language comes along which is not someone's favourite pet that will get the blame.