wlan card (AR928X, ath9k) slow since upgrade to natty beta 2

Since upgrade to Natty my wireless card is not working at full speed anymore. The card itsself seems fine as it reaches full speed when booted under windows. Also, it does not seem to be a general network problem, because the LAN connection is fine. The card connects, but download speed is awfully slow.

decker@Thor:~$ sudo lshw -c network
...
*-network
       description: Wireless interface
       product: AR928X Wireless Network Adapter (PCI-Express)
       vendor: Atheros Communications Inc.
       physical id: 0
       bus info: pci@0000:06:00.0
       logical name: wlan0
       version: 01
       serial: 00:16:44:60:10:55
       width: 64 bits
       clock: 33MHz
       capabilities: pm msi pciexpress msix bus_master cap_list ethernet physical wireless
       configuration: broadcast=yes driver=ath9k driverversion=2.6.38-8-generic firmware=N/A ip=192.168.0.126 latency=0 link=yes multicast=yes wireless=IEEE 802.11abgn
       resources: irq:21 memory:f0400000-f040ffff

decker@Thor:~$ sudo ifconfig wlan0
wlan0 Link encap:Ethernet HWaddr 00:16:44:60:10:55
          inet addr:192.168.0.126 Bcast:192.168.0.255 Mask:255.255.255.0
          inet6 addr: fe80::216:44ff:fe60:1055/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
          RX packets:19836 errors:0 dropped:0 overruns:0 frame:0
          TX packets:21548 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:18904616 (18.9 MB) TX bytes:3354995 (3.3 MB)