The router speed is a mere indication of router capability of transmitting the data. It does not mean faster internet speed. The internet speed is limited by the ISP provider. More precisely the internet service provider, your selected package and FUP limit in case you have
The faster router will help boost the speed of sharing the data in the internal network, called as LAN. Say one of your family member or friend wants to share a movie on a router. You dont need internet for that. You can share it over the router network. So this sharing speed depends on the router speed capability 150 Mbps or 300 Mbps? 300 Mbps router can share the data twice faster than 150 Mbps router. That simple. In another perspective, we can say router speed is the speed between router and computer and not internet speed.