I have a Linksys E4200 router and a laptop with an Intel Centrino Wireless-N 135 adapter. If I place my laptop right next to the router, the link speed is about 20 Mbps. Is that normal or is there any way I can increase it? (P.S. Sorry if this question sounds stupid...)
No, it is not normal if the router is operating in 802.11n mode. It might be that the antenna in the router is disconnected, but first you should play around with settings of the router.
Make sure that the router is in only 802.11n mode. 802.11n/g is also good, but not 802.11b, which usually causes routers to lose performance for ALL devices. In the end, it would be smart to reset the router to see whether that helps.