WAN : The router will only connect to the internet at the best speed possible for your line. This will be negotiated with the switching equipment at the exchange. The router is capable of connecting way faster, but it is limited by the line quality.
If the router attempts to connect faster than the line quality will allow, the result will be lost packets and this will slow down throughput.
LAN : The router has an internal switching hub, allowing it to be share the internet connection with several machines. This will normally connect to the other machines at 100 Mb/sec and changing to a different brand will make no difference.
The only reason you may be experiencing a slowdown (assumed to be your internet connection which is slow) would be if the router settings are different and / or wrong. Other than this there is no real difference in the connection speed capability of different brands of router.
What sort of speeds are you getting ?