Very slow speed. Router problem. Any advice?

Modem (Netgear CM 500)

Router (Linksys E3200, up to 300 Mbps)

I used an old computer to link directly to the modem. The speed was approaching 300Mbps.

How can I modify the router to get a higher speed?

Since I have a modem, I don't want to spend more money on XFi each month. Any advice on purchasing the Router?