I've just got an RPi with a pre-installed OpenElec XBMC SD card and have it set up for audio streaming to my sound system from my NAS. I initially set it up using wired ethernet but have now moved it on to wireless and experienced some really wierd behavior.
To do the set up I had a USB mouse plugged in alongside the WiFi dongle and everything worked perfectly. Once I finished I powered the RPi down and unplugged the mouse & monitor, then powered it back up. The performance was so awful I couldn't play anything and it took about a minute even to log in to ssh.
I thought I had done something wrong so powered it down again and brought it back up with the mouse and monitor plugged in again. When it came back up it was once more working perfectly.
After a number of experiments I found that if there is nothing plugged into the second USB slot the WiFi dongle (plugged into the slot furthest away from the board) just goes mad. Plugging a mouse or USB memory stick into the other slot allows the RPi to work perfectly, playing for hours without a single stutter even when I was browsing the library via the Constellation remote.
I haven't been able to find any documentation about which USB slot to use if only 1 is needed. Has anybody else experienced this behaviour?