Disable NAT boost is the first thing I tried to turn off, unfortunately, it won't work (also tried to restart and turn off other features). Only after I downgraded firmware everything working ok, CPU...

@Metallicafrk87 I just downgrade router firmware from Archer C9(US)_V5_190403 -> Archer C9(US)_V5_180423, the router seems working now. I'm not sure if it actually fixes the problem.

@Metallicafrk87 I have the same problem with my C9 v5. I only notice the problem after plugin my USB to share data. Tried use all the FTP client I have on pc, laptop, iPad all the client hanging...