This will largely depend on how much (read: bandwidth) you want to route. I used to be a big fan of the PCEngines ALIX boxes (https://openwrt.org/toh/pcengines/apu) because you could run pretty much anything on them, but with big gigabit connections nowadays, they're less well suited.
If you're looking to route the lower end of things, you're likely to be fine with an SBC like the above. If you're looking for more, then something with Hardware offload is worth looking out for. With OpenWRT, you're likely to be looking at either a fairly meaty X86 for gigabit, or an off-the-shelf for which it supports HW offload.
I have a Celeron J4125-based fanless mini PC which I run OpenWrt on. It has been fantastic for me and can route my 1000/25 connection at 100% without breaking a sweat.
It's a Qotom Q750G5. Similar models can be purchased from Protectli if you do not want IME or if you want coreboot.
Anyway, it is a fantastic little device which sips power and I'm very happy with it.
Edit: one last note -- I use this for routing only. I did not add the wlan module since I have dedicated access points installed in some of my closets.
Thing is that most users want some form of qos/aqm to guard against load induced delay, which the apu and similar low power devices cannot handle above a few hundred mbps
That's interesting, because you've identified the reason I asked the question - on my TP-Link router wired connections just flood out wireless connections, and make them unusable as long as I'm doing anything heavy (like downloading ISOs).
The solution's not going to be found in the firmware, is it?
Yes, in general, having a better router would probably help (although you haven't mentioned the exact model you have, maybe it's already fine and the problem is elsewhere).
Enabling SQM [1] would probably mitigate the issue further, at the cost of requiring more processing power.
I found this to be completely unnecessary when I moved to a 100Mbit symmetrical connection. Not sure if its the higher speed or having a better upload channel, but I completely got rid of all QoS/traffic shaping on my router and never felt the need to turn it back on.
The need for aqm is dependent on the status quo. Most providers nowadays use red/pie in the last mile and buffers-inducted delay rarely exceeds 100ms; much less so when it became clear that flow-queueing would be required and possible. Also multiple queues with codel on them have found their way into some wifi stack’s to deblot this access layer
If you're looking to route the lower end of things, you're likely to be fine with an SBC like the above. If you're looking for more, then something with Hardware offload is worth looking out for. With OpenWRT, you're likely to be looking at either a fairly meaty X86 for gigabit, or an off-the-shelf for which it supports HW offload.