Because the performance of existing web frameworks is... _absymal_. They are nowhere close to how fast they can be. And I don't mean slower by a bit, I mean close to a factor of 10x. My in-house framework can serve 600-700k requests/s on a single core, while beast (the best c++ framework i could identify) does 250k at best. nginx/apache and other prebaked servers are even slower, at 100k req/s. Tells you all you need to know.
Performance is probably the only angle that has some potential but only short requests, the db and other external IO/services is a bigger bottleneck.