There are two speeds you need to consider.
The first is the speed at which the router connects to the ISP over the phone line. This shouldn't change much and is the speed the ISP will have quoted as up to xx Mbps.
The second speed is the connection from your laptop (or other device) to the router using wifi or an ethernet cable. With a cabled connection it would normally be 100 Mbps or 1000 Mbps (also described as 1Gbps). For wifi the speed will vary depending on the wireless signal quality. The wireless signal quality will be affected by lots of things including parts of the building. Use of Microwave ovens, use of cordless land line phones as they all work in the same frequency range.
To improve the wifi signal is some areas there are a couple of things you can try. The cheapest option would be to move the router to somewhere more central. If there are still some areas not covered look at getting to Wireless Access Points and put them in areas with poor coverage and connect them back to the main router with an ethernet cable. With wireless Access Points (rather than routers) you'll be able to have them use the same wireless network name and password as the main router and you should easily roam from one to another. (If you got Wireless routers instead they create their own network so they need differing names so the device knows it's moved to a new network).