example: Both Routers have massive traffic going through...
Router 1: cheap hardware, basic model
Router 2: Middle to top of the line hardware
Router 2 is going to be able handle more traffic through put because it has better hardware than Router 1. Since it can handle more constant connections, it would appear that there would be less lag, but in all reality it can handle more connections.
Not all expensive "top of the line" routers will spank the cheaper stuff... I use to use a cisco small business router $200, switched to the D-link gaming router $89 (at the time) and the D-link spanked the hell out of the cisco router....
Also there are other bottle necks that create what looks to be "lag". GFX driver out dated, slow HDD, slow RAM, viruses, fragmented HDD... High settings on an old graphics card, the list could go on and on.
Hopefully this helped out a little.