In general I don't see any reason to look at any data in the payload before the signature is verified. If anything, a good reason is that increases attack surfa

In general I don't see any reason to look at any data in the payload before the signature is verified. If anything, a good reason is that increases attack surface by giving attacker ability to interact with my system without the signature. I don't know what exactly you mean by improving performance by looking at the payload before the cost of signature verification. If you mean that for some reason you could discard large portion of messages without actually verifying the signature then this is generally false. In a DoS situation the attacker might want to consume your resources without bothering to forge the signature but just as well he can forge the content so potentially easy fix to their fake content will cause you to pay for the signature verification anyway. If, on the other hand, the application somehow relies on this to be viable then very likely this is a design fault. I can imagine for example that you might want to discard messages with outdated tokens but there are better ways to solve this (just don't send outdated tokens?)