I don’t have any experience with an APU2, but I’ll ask this:
Does your ISP use PPPoE? People never, ever mention this in their posts, yet it’s a huge factor in performance. Having to encapsualate and de-encapsulate every since packet egressing/ingressing into your router will have a big performance impact.
You could check if this is a device limitation by looking at top/htop while running your tests, is the CPU hitting 100%?
There’s some performance tuning notes here.
There’s also notes here about turning off the meltdown/spectre workarounds for extra performance, but reading this post seems to indicate that maybe 600mb/s is the limit of the device (assuming you ARE using PPPoE)
Hope that helps a little?
PS: Thanks for supporting the project as a Patron. (I am not a project member, just a happy user)