x86.renejeschke.de | X86 instruction set reference

The movzx instruction zero extends a quantity into a register of larger size. In your case, a word (two bytes) is zero extended into a dword (four bytes). Zero extending itself is usually free, the slow part is loading the memory operand WORD PTR [rsi-2] from RAM.

To speed this up, you can try to ensure that the datum you want to fetch from RAM is in the L1 cache at the time you need it. You can do this by placing strategic prefetch intrinsics into an appropriate place. For example, assuming that one cache line is 64 bytes, you could add a prefetch intrinsic to fetch array entry i + 32 every time you go through the loop.

You can also consider an algorithmic improvement such that less data needs to be fetched from memory, but that seems unlikely to be possible.