Basically, my PC is getting anywhere from 0.4mbps-10mbps when other devices are getting 55-60mbps. I just bought a new wireless USB adapter thinking mine had quit on me but it's not helping either. Basic specs are as follows:
AMD FX(tm)-8320 8 core 3.5GHz
windows 8.1 64-bit
NVIDIA GeForce GTX 960
Linksys AC1200 dual band wireless-ac USB adapter
Have you tried connecting your PC using wired connection and if so what speed do you get? If you're getting normal speed using wired connection it can be a problem with the wireless USB adapter that you're using.