Long story short, I suspect my Archer C6 is throttling internet speed :/

To be clear: I'm not talking WiFi - all measumenets below are taken over Ethernet connection.

So, my network setup is as follows:

               .----.
   .---------. | == |
   |.-"""""-.| |----|             |   |
   ||       || | == |         \  _|___|_  /          _______
   ||       || |----|          \:       :/          :       :
   |'-.....-'| |::::|=== Eth ===:_______:=== Eth ===:       :-- Fiber --
   `"")---(""` |___.|                               :_______:
  /:::::::::::\" _  "            tp-link            
 /:::=======:::\`\`\            Archer C6          Fiber optic
 `"""""""""""""`  '-'                                 modem
     Desktop           Cable 1             Cable 2

What I've recently found interesting is that PC shows down/up speed of 255.56/264.11 (Mbps; link). This is way lower than the incoming (>1Gbps) and network cards/router physical limitations (both 1Gbps).

I tried connecting cable 2 directly to the PC - and it's surprisingly 363.86/907.48 (Mbps; link)!
900+Mbps looks somewhat close to what I've expected... But why doesn't it work with Archer C6 in the middle, which claims to be a Gigabit router? I've already tried replacing Cable 1 with other cables, but the result is still the same.

One thing I've noticed in addition is that router CPU load is at 100% during speed tests. That may mean router can't handle more... Now the question is, what settings may I tune to stop throttling? Given I have QoS, Parental Controls and Security all disabled, I'm pretty lost on what else to turn on/off.

ps: I have both 2.4 and 5Ghz wireless networks enabled with some 6-9 devices connected, but they are mostly idle. And switching wireless networks off completely doesn't actually affect Ethernet speed still - already tried.