even with a good connection, equipment variations will cause differences in the speed you get as will walls, furnishings, distance, other signals, etc. not to mention any issues your ISP may have on their end
If Wifi isn't running at an acceptable speed you should look into direct connection with the modem. This would require you to run Cat5 (or Cat6/7) through the walls and directly to whatever device you want on the network. There are limits to this as well based on the distance but anything under 100m of Cat5 shouldn't see much noticeable loss of speed and would still be faster than WiFi generally.
So basically wherever your router is you should have as many Cat5 cables going out from it as you need and hooked into the ethernet ports of whatever devices (playstation, computer, TV, etc.)
Running Cat5 through the walls isn't very hard. You will just need a length of Cat5 (cost depends on length but generally very inexpensive), a drywall saw (couple bucks at a hardware store), a fish tape (no more than $20 at a hardware store), wall plates if you want a neat finished look (from $0.96 to quite a bit more if you want to add USB, HDMI, other goodies), a flashlight, a partner if neccessary, and some patience.
just make sure you use in-wall/plenum rated equipment, keep to local fire codes, and don't cut through fireblocks and fail to fill them after you are done. You may also consider putting in a conduit if you think you will ever add more cables. That way the next time you won't need a fish tape at all...