A few general pointers for WiFi that will help you out …
As some of the other people that have responded in this thread have mentioned some of these points, I've collected them here:
Signal strength at BOTH ends is important. How well you see the AP is only half the puzzle. If the AP doesn't see the client equally as well, then its not going to work well. Remember that an AP doesn't have the physical limitations of an antenna that'd you'd find on a tablet, meaning the AP's antenna probably works better than the tablet's.
Signal attenuation is a killer: walls, crappy antennas, etc. If you have any way of measuring the signal, you want at least -70dBm (yes it will still work, but poorly at -80dBm) from the AP at the client and vice-versa. Also don't run your APs without antennas. The radios inside are impedance matched to the antennas, disconnecting them generally makes it not work beyond a few feet.
Interference is a killer: Make sure you are on your own channel, in 5Ghz, that's possible, but avoid DFS channels (52-144) since by law the AP must switch off and find a new channel if it detects radar. As an additional point, if you run custom firmware in your AP, it may be tempting to select certain non-standard channels, just be prepared for a visit from law enforcement agents, big fines and equipment seizure especially if you're near an airport!
Most people have no clue about 2.4GHz: Stick to channels 1, 6 and 11 (or whatever is standard in your country). Otherwise you're just creating unintelligible noise for your neighbors, which is far worse than interference on the same channel, which can at least be understood and respected by all parties.
Please for the love of God don't run 802.11n 40MHz in 2.4GHz, you're just being a pig since there's only 1 usable non-overlapping 40MHz wide channel in 2.4GHz.
More power != better performance; in fact in many cases more power = worse performance. Think of how you'd want to install speakers in a whole home audio system. One super powerful speaker in a central location, or many smaller speakers scattered throughout for a pleasing uniform sound level everywhere. You can also use the same analogy for lighting. Same thing applies to WiFi, RF waves are like light waves, only they penetrate somewhat the obstacles.
Don't forget WiFi is a half-duplex medium: The radio can only send or receive, it can't do both at the same time AND each and every wireless packet has to be acknowledge or retries occur. Consequently, your expected performance will be about half the connection rate, if that.
Consider using iperf to validate your throughput. Run iperf in both directions to ensure you're getting what you're expecting.
In the end, unless you have the budget for Enterprise grade wireless, you have to work within with the above realities.
–A.