to /boot/config.txt and reboot. Maybe also enable software offloading in the firewall. Beyond this there could just be something aberrant in the install. RPi4 can and should be able to route close to a gigabit without SQM and VPN. I'd recommend wulfy's build for a solid base.
thanks for your help. (i had already activated software offloading)
those config settings didn't help.
i'll try installing wulfy's build when I have some permissible network downtime to do it.
I use a snapshot from mid-march 2021 as my default router for gigabit fiber WAN and several gigabit internal VLANs. Straight out of the box and without tuning I can get simultaneous gigabit throughput across several ports. With tuning (raising the minimum CPU freq to 1GHz, manually distributing the interrrupt core affinities with a script) I get that without the CPU ever going over 10% on any core.
I agree with what @anon50098793 says above: your problem is probably not Pi related and I don't think using one of his builds is going to fix it. What kind of switch is it connected to?
Don't enable software offloading on the Pi - only use irqbalance (and make sure it's actually working - counters for IRQ 31 should be on CPU 1, IRQ 32 on CPU 3, and IRQ 39 on CPU0),
You need to check the computer you're using for the test and the Ethernet cable it's using.
The Pi itself, if nothing is borked, can route and SQM a gigabit so it's not the bottleneck unless there's a cable issue or it's got a physical problem like an altered CPU clock speed limit
It's either a USB2 dongle, or it's on a USB2 port.
Easiest thing is to unplug and then re-plug making sure to put it in one of the BLUE colored ports. If it still says Spd=480 then it's not a USB3 dongle.
speedtest through xfinity (my isp): says i'm at my paid-for speeds of 600 mbps speedof.me: 100 or so mbps on wired, ~250 mbps on wireless.
iperf3 on the router: ~28 mbps
iperf3 on network machine (wireless): ~20 mbps