My max throughput when the cable modem is connected to the router is ~150mbps. If I disconnect the router and directly connect cable modem to a computer, I get 500mbps. After some investigations, apparently the culprit may be the router. Specifically, OpenWRT is causing the bottleneck (some sources online suggest that returning the C7 to stock firmware resulted in max throughput of ~600 mbps)
Is that true? Is the router the most likely cause of my speed bottleneck?
If so, can you suggest a price-conscious router that will work with OpenWRT but that will be able to handle gigabit internet speeds?
I wouldn't go that far, older x86_64 systems (unless carefully selected) tend to have a rather high idle power consumption (75-130 watts, 24/7), getting a low-power device (can be x86_64, but doesn't need to be) quickly pays for itself (6-15 months, due to lower electricity costs).
Put it in an enclosure, and add a power supply to get a two-port Ethernet router that can run OpenWrt at GBit speed (according to the original 500Mbit+ post). You can keep your Archer C7 as a dumb access point since it has decent Wi-Fi.
This gives you a pretty high-performance router with relatively low power consumption at a very modest price. Have fun!
A simple Pi 4 + UE300 will achieve the same thing without the freakishness of the Compute Module, but either way you'll also need a (preferably smart) switch. It will indeed do shaped gigabit and gigabit Wireguard.
Depends where you live. That's because availability of anything OpenWRT varies from region to region. In my case i have the same router as a dumb access point with the original firmware, and a RPI4 2GB model routing, doing SQM, print server and small file share unit, although i don't use the RPI4 WIFI capabilities. I never got more than 25-30% usage and consistently reach 900+ mbps (on the Archer C7 i usually use only the N network capability, due to smartphone usage here). Can't tell how well will perform with VPN enabled, since all RPI's have a Broadcom processor without hardware accelerated encryption.
If you can't find a good replacement for your Archer C7, maybe you can give this setup a try. (Caveat emptor: i did wrote a guide for a custom build, that can be used for the 21.xx release, but still you need a usb-ethernet gigabit dongle compatible with linux.)