The biggest factors that reduce Wi-Fi performance are the effect distance and internal walls have on signal strength.
Make sure the radio waves don't run into any obstacles. Everything blocks Wi-Fi signals a little. Wood, plaster, cinder blocks, and glass don't interfere much, but brick, stone, and water can be more problematic. Worse still are ceramic, concrete, metal, and mirrors.
When the signal passes through a thicker wall the signal is degraded. Moving the device to another part of the room, where the signal just passes through thin walls, improves the signal.
In some situations you can get 'pockets' of good and bad signal quality.