Performance against Python: find substring in string

[Araq]

Well you expect 2-4 times better performance for a program that spends all its 
time in the Python interpreter's C code. And that's simply unreasonable. In 
addition to that you use a severely outdated Nim version and Nim's `find` 
operation uses the Boyer-Moore Horspool algorithm which is typically worse than 
an SSE brute force solution would be.