That is question that is actually much more complex than you think. It likely is actually how many packets per second the device can pass and not how many mbps. The packets could be anywhere between 64-1500 bytes in length.
Still even very inexpensive routers can get 900+mbps on speedtest sites.
Now that is only if you use the router as a simple NAT router. As soon as you use other features the speed can drop a lot. Using things like firewall features or vpn takes a lot of cpu power.