Hashcat only using ~2% of GPU?

[epixoip]

GT 750M is basically an underclocked, low-power variant of a GT 650. Back-of-napkin math shows that it should get around 13 KH/s, so the 12 KH/s you're seeing is certainly correct.

Whatever you're using to monitor the GPU in Windows Resource Monitor, on the other hand, definitely is not working. Have you tried GPU-Z or MSI Afterburner or something?