CPU starts to matter once you start doing anything interesting outside of the default configuration or if you really do have very high bandwidth Internet connections.
QoS can be CPU heavy at the limit, but if you are only using it for low bandwidth applications like gaming, you almost certainly won't notice. The quality of the software, e.g. what type of QoS, how flexible and accessible, would be more important. The
standard QoS advice from Ubiquiti applies:
- ERLite-3 and ERPoe-5: below 60 Mbps most likely will work, above 200 Mbps most likely will not work.
- ER-8: below 160 Mbps most likely will work, above 450 Mbps most likely will not work.
- ERPro-8: below 200 Mbps most likely will work, above 550 Mbps most likely will not work.
- ER-X and ER-X-SFP: below 100 Mbps most likely will work, above 250 Mbps most likely will not work.
The new Edgerouters like the ER-4 are more like "below 200 and above 500".
CPUs start to matter for: VPNs, large number of simultaneous connections (especially small packet sizes), extra services like file server/NAS, large number of firewall rules, etc.
A 700 MHz 3x3 router with good radios, or a 600 MHz wired router + couple of 2x2 APs will cover many consumer Internet needs.
Keep in mind that these machines are more or less mini-PCs dedicated to specific purposes, rather than general purpose machines. That means if you do not aggressively use those purposes, you won't notice any difference. It also means that the more you pay for a router, the better the comparison to a custom PC build since at higher price points you'll get much more bang for your buck - at this time, basically around $300+ is when it becomes more than silly to buy off-the-shelf consumer routers, unless you are someone who cares far more about convenience at any price point than anything else (but then you wouldn't be posting on a power user forum...).
Ultimately though, hardware reliability, receiving updates, especially security updates, and flexibility, tends to matter far more than all of the above put together. Good luck finding such a combination that fits your needs in consumer routers (the Ubiquiti's, Mikrotik, enterprise Cisco, Juniper, HP, Ruckus' of the world don't count, even if they can be behind in consumer features).