ER-Lite has a really weak CPU, and can only get close to 1Gbps using its hardware offloads, which limit what you can do to the traffic passing through the router. ER-X has a faster CPU and can get reasonably close to 1Gbps with software packet forwarding for simple rules, and can handle traffic shaping at far higher speeds than the ER-Lite (though neither can shape anywhere close to 1Gbps).