WNDR4700 slow WAN speed

Hi there,

I have a bit of a problem with my WNDR4700. On my Gbit/s WAN connection (ISP claim) it gets about 150MBit/s. The connection to the modem says Gigabit.
Regular LAN transfer speed is Gigabit (measured with iperf3). However WAN speed is ~150 MBit/s. I tested the router on a different connection with a different modem of a neighbour (he gets 350 MBit out of a much older router) - my router still has only 150 on the other connection.

I read this post: ASUS RT-58U slow WAN speed and it reads the same, but there is no solution.

What I tested, all connections wired:

  1. measure WNDR4700 my modem with iperf3 against a public server: 150MBit/s
  2. measure WNDR4700 neighbours modem with iperf3 against public server: 150MBit/s
  3. measure neighbours router neighbours modem with iperf3 against public server: 350 MBit/s
  4. tried 1. with a reassinged LAN4 as WAN port: same as 1.
  5. measured LAN speed to rule out bad LAN: 1 GBit/s
  6. Observed htop, CPU load spikes to about 50% during tests @150MBit/s so there is still room left

I am already running latest 19.07.2.

Where can I look into the router what is happening? I think I remember getting faster speed sometimes, but the test with my neighbour would rule that out...

Thanks a lot, I am at a loss