I used that library for some tools that I made but it is not optimized for secp256k1 also it is some kind of vulnerable to some side channels attacks and incomplete because it declare EC.b parameter but it never use.
A lot of improvements can be made to that implementation.
The fastest implementation for secp256k1 code that I ever see and use it is already inside of kangaroo tool.
https://github.com/JeanLucPons/Kangaroo/tree/master/SECPK1Same library that I actually use in my keyhunt code.