Question

Which is more CPU intensive, to do an if(x==num): check, or to do a sum x+y?

Answer 1

Your question is somewhat incomplete because you are comparing two different operations. If you need to add two things together then testing x==y isn’t going to get you anywhere. So presumably you want to compare

if y != 0:
    sum += y

with

sum +=y

It’s a lot more complex for interpreted languages like Python, but on the hardware a test for non-zero introduces a branch and that in itself can be expensive. But I wouldn’t want to say which would be faster without timing.

Throw into the equation different performance characteristics of different architectures and you have another confounding factor.

As always, you are best to write your code in the most natural maintainable way first and then time it. If you feel you need to extract more performance use a profiler to find hot spots and then optimise.