[Solution found] Perf regression due to sudden extra allocation for result values
benchmark "ex01_xor" used to take 17.30s on my single core i5-5257U

It now takes over 18.5 sec.

This commit which reworked GEMM is suspected: https://github.com/mratsim/Arraymancer/commit/ceabd73360d991d6b011d898f07207874ec1d27d