PHEW, solved it!
Apparently Geekbench has a separate AVX2 executable on Linux, which performs much closer to expectations. #Linux ftw, as it should be...
@killyourfm why is the frequency lower on the windows install?
@necrophcodr @killyourfm probably one's taking the base and the other the boost one, if I had to guess due to different behavior of platform-specific code.