I'm trying to set it up on a RPI4 as an 802.11ac wireless router, to verify this. If it manages 100mbps+ then it'll be a cheap replacement for my current router.

Are you keeping a write-up of your progress anywhere? Would love to see how this turns out.

Not yet; but that's a good idea. Thanks for the nudge :)

are you using OpenWRT or something else?

Starting with Ubuntu for now, and hostapd.

OpenVPN is extremely slow, same setup with wireguard is guaranteed to be much faster.

Raspberry Pi 3 can do at least 250 Mbps. Perhaps it's limited by something else, like crypto or your actual connection?

Not the original Pi 3B, you're thinking of the 3B+. https://magpi.raspberrypi.org/articles/raspberry-pi-3-specs-...

I just ran iperf from my Pi4 to my Ubuntu VM in the same LAN five minutes ago. 850 Mbps symmetric with no VPN.

I get 940 Mbps for two devices on the same switch.

Which bus limits to 60mbps? The USB limitation shouldn't be anywhere near that low.

I don't know, but you get the extra limitation of USB2 being half duplex. USB2 is something like 480mbps. Add half duplex limitations and just general overhead compared to theoretical max. I can see how some network protocols become limited to 60mbps.

The USB bus on those is shared with all devices

True. It does depend what else you are doing with the pi. Something like a USB hard drive could account for that poor performance, especially if you were upload from/downloading to it.