Thanks for the explanation Nikita. Any branching, hashing etc will increase execution time. The execution example is on laptop, which has 2 memory channels. 0.13s is absolute max that this laptop is able to muster as far as memory throughput goes. Usually 4 threads is enough to saturate memory.

I have written this piece of code: https://github.com/questdb/questdb/blob/master/core/src/main...

This sums 64bit values and using AVX2 it will sum 1Bn in 0.26s. Incrementing conditionally will not be as fast and will throw vectorization out of the window too.