Benjamin_24 :
So I share wifi with my neighbors and their router and modem are on the second floor. I live on the first floor with my pc right below their router yet I still experience lag spikes during gaming sessions.
The little whip antennas on routers are only omnidirectional horizontally. Their signal strength forms a torus (donut shape). Directly above and below them is where the signal strength is weakest when the antenna is pointed straight up/down.
http://mpantenna.wpengine.netdna-cdn.com/wp-content/uploads/2015/02/FIGURE-1.png
Before resorting to buying new hardware, you can try tilting one of the antennas by about 45 degrees or whatever it takes to intersect your PC. (If the router only has one antenna, this may not be an option as it will affect your neighbor's reception as well.) The antenna on your PC's wifi adapter should be tilted at the same angle - reception is best when the sending and receiving antennas are parallel to each other.
If this is an older 802.11b or g router, those protocols would slow down to the speed of the slowest connected device. 802.11n and ac support different connection speeds for different devices. However, if you have a n router which supports b and g, and a g-only device connects to it, it may put the router into g mode. Basically make sure all the connecting devices support n or ac (in the router's wireless settings you can disable support for b and g, and see if anyone complains their device no longer connects).