Ethereum is not slow (tps) because it has to make difficult computations. Ethereum is slow because it needs to be fair to all participants considering the following issues:
1) latency. All data needs to be distributed to all nodes
2) coherency. All participants need to agree on the data and it's ordering (consensus)
3) distributed nature. No centralized ...
Yes, if you have forked then you are free to change the consensus code.
The consensus part is mostly in the consensus/ directory of the https://github.com/ethereum/go-ethereum repository.
I don't know of any guide to make modifications to geth source. Since the code is always changing it is likely that such guide will be obsolete almost immediately.