Resizable arrays in Nim - difficulty in getting performance

[demotomohiro]

If your code is compiled for x86_64 and SIMD instructions are used, xmm, ymm or 
zmm registers are used. But these registers being used doesn't means multiple 
data are calculated with one instruction. You can search x86 and amd64 
instructions here: <https://www.felixcloutier.com/x86/>