what are the ways to make the autorouter in eagle work faster?
Apart from the solution which you already aware about - using faster PC - there's a way how to make it running faster, but it is close to manual routing.

Autorouter should router faster when you place components in convenient places of the board making autorouter's task easy, but, to do it properly, you need to have idea about how board should be routed. But if you know how it should be routed, you will be able to route manually relatively effectively and efficiently.

There're several tips setting up component layout for routing - most obvious is having - tracks at one level and | tracks at another level, having them connected through through-hole pads or vias: this can be implemented very well for set of wires having same function (e.g. address bus, data bus).

In my opinion putting restrictions on routing area should slow down the autorouting task - simply because there're more components for autorouter to consider during routing.