I don't have any throughput problems with my setup. From pingtest.net, I consistently receive the following results from my 6 month old PowerAP-N:
Packet Loss 0%
Ping 34 msec
Jitter 2 msec
Grade A
From speedtest.net, I consistently receive the following results:
Ping 21 msec
Download speed 5.09 Mbps
Upload speed .65 Mbps
I get those numbers about 200 feet away from the AP-N and with the wireless signal going up one floor, penetrating three walls and also going through several mature trees in the yard that separates the PowerAP-N location from my principal client locations. My performance results are also pretty consistent with a Mac Airbook, a Mac Powerbook, and a Mac IMac.
My signal source is a Zyxel DSL modem that is hard-wired to the WAN Port of the PowerAP-N. My service is ATT Elite DSL, which claims 6 Mbps down and .5 Mbps up speeds. I have verified with the same Mac portable clients that were temporarily hard-wired to the PowerAP-N that the above download and upload speeds are being limited by the ATT Elite DSL service and not by the PowerAP-N's wireless speeds. I also tried a direct Mac portable Cat5 connection to the Zyxel modem and did not notice any appreciable speed changes with the PowerAP-N completely out of the system; I don't have those direct connection, all wired, numbers handy right now.
I have 4 other interfering 2.4 Ghz networks in my neighborhood. They used to wreak havoc with a Cisco E4200; I had to reboot that devil several times per week to maintain acceptable and stable wireless throughput speeds. The PowerAP-N deals with those competing networks without any problems. I have a software firewall configured in my Mac computers. I did not try firmware based firewall rules in the PowerAP-N itself. Are you using any firewall setups?
As Stevech suggested, you should try to eliminate variables by trying a different client(s) with your PowerAP-N and report back to us with your throughput and ping/jitter findings. If you see wireless speed and ping/jitter issues that are significantly slower than your wired speeds, you can also use the PowerAP-N's RF spectrum scanning feature to identify whether or not you have signal interference problems with nearby and competing networks. My PowerAP-N just seems to overwhelm the nearby networks with its much stronger transmit signal and 5 dbi gain antennas for both transmit and receive directions; on the receive side, those vertical omni antennas help significantly with weak client signals from laptops and with lots of obstacles between the transmit and receive devices.
Also take a look here, as PowerAP-N users have not reported speed limitations like you are experiencing:
http://forums.smallnetbuilder.com/showthread.php?t=5312