Here's some reference I've searched:
https://softwareengineering.stackexchange.com/questions/49550/which-hashing-algorithm-is-best-for-uniqueness-and-speed

Every operator need to call Findvar directly or indirectly.

As C++ default hash<std::string> implementation uses "murmurhash", which is the fastest hash function, it may still slower than directly lookup a variable using an offset. We can consider a way to make runtime Findvar implementation do not use unordered_map.