Debian Configuration :: Slow And Laggy Network With AR5008 (Ath9k) - XP Okay

I got a TP-Link WN951N Wireless-N PCI card for my Squeeze HTPC (this is AR5008. Performance is awful. On the lucky runs I'm getting 4 MB/s from Samba, most of the times I'm seeing 1-2 MB/s, and sometimes less than 1 MB/s. Also, ssh is not smooth, e.g. when I type commands, it can take seconds before text appears on the console, or I get slo-mo. Forcing 11g gets me a consistent 2.2 MB/s but lag is still there.

The machine has also been tested under XP SP3 and pulled off a solid 11 MB/s on a large file with no lag, so the hardware would seem OK. I've tried several kernel versions (both Liquorix and Debian stock), and a couple versions of compat-wireless (2.6.38-rc4 and 2011/03/03 bleeding edge), with little variation in outcomes. The logs got no weird messages in them, although with the more recent drivers/kernels iwconfig looks like this:

wlan0
IEEE 802.11bgn ESSID:"GuessIt"
Mode:Managed Frequency:2.457 GHz Access Point: x:y:z:t:u:w
Bit Rate=270 Mb/s Tx-Power=19 dBm
Retry long limit:7 RTS thr:off Fragment thr:off
Encryption key:off
Power Management:off
Link Quality=46/70 Signal level=-64 dBm
Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0
Tx excessive retries:64 Invalid misc:19967 Missed beacon:0

Tx discarded packets are through the roof and excessive retries is not bad either. The system doesn't run NetworkManager, wicd or whatnot. All of the wireless is configured through /etc/network/interfaces:
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
allow-hotplug eth0
iface eth0 inet dhcp
allow-hotplug wlan0
iface wlan0 inet dhcp
wpa-driver wext
wpa-ssid GuessIt
wpa-ap-scan 2
wpa-proto RSN
wpa-pairwise CCMP
wpa-group CCMP
wpa-key-mgmt WPA-PSK
wpa-psk <hexbabble>

Did I commit some horribly naive mistake or overlook something Really Important(tm) ?