Netgear R7800 only getting about 140Mbps down - why so slow?

Instructions to disable power savings on Ubuntu/Mint (will work for you too)

Roaming agressiveness, instructions, these may or may not work.

Also are you compiling your own build, using the stock images or using hnyman's build? I would try with hnyman's build, or try compiling using his build environment.

hnyman's builds: