The further your device is from the router, the more your WiFi signal degrades, and therefore your speed, too. The severity of this effect highly depends on your home construction, location, surroundings...actually a lot of things.
The WiFi signal from your antennas is broadcast in all directions, so it's best to place your router in a central location in your home. A higher location is typically better.
Walls, water, metal, and mirrors are enemies to your WiFi signal. Avoid placing your router near them and don’t expect your WiFi to get through them very well.
Avoid the Kitchen:
Kitchens are full of metal and microwaves; avoid it where possible. Often the wireless phone base is in the kitchen, too. Phones may use the same frequency as your WiFi, so avoid placing your router near one.
Adjust the Antennas:
The antennas receive and broadcast WiFi. Their orientation can make a big difference. Try moving them around to improve the signal in different areas of your home. The signal is oriented perpendicular to the antenna. So, for example, if you want a better signal horizontally, orient the antenna vertically. You should also be aware that some antennas are located within the router. To improve signal from them, you will need to experiment with the orientation of the router itself.
SEE ALSO: Using extenders with Router Limits