I've been suffering from very slow wifi performance from my iPad 2 in parts of the house where all other products work fine (iPhone 3GS, Samsung Laptop, Asus Netbook). I use a Draytek Vigor 2800 VG router which NEVER lets me down.
I've tried all the usual fixes (only using 11g, encryption/no encryption/different types of encryption, power down, forget network, renew lease, adjusting brightness, turning bluetooth off, etc etc) none makes any discernable difference to performance in areas of weaker signal.
I've been testing my iPad 2 further this morning, with / without smartcover and in various different parts of the house, I've been using the speedtest.net app to test down / upload speeds the results and some explanation are as follows: -
1) The iPad and router are communicating fine, there's no periodic dropout of signal or signing on issues when in close proximity to the router - effectively working fine with full signal strength (little or no buffering of videos)
2) When I move to other parts of the house where other products get no problems (laptop, netbook, iPhone) but signal strength is degraded due to walls etc, the iPad 2 still receives wifi signal but performance is greatly reduced (download speed of 0.33 Mbps in comparison to the iPhone at 5.33..... rural Oxfordshire this is the best you can hope for!)
3) If I move slightly closer, remove some obstacles (walls) between me and the router, the performance picks up fine, 5.77 Mbps for the iPad and 5.20 for the iPhone.
4) The smartcover makes little or no discernable difference to performance eg. cover on 5.72 Mbps vs. cover off 5.77 Mbps in same location
In conclusion, 1) either there is a software issue with the way the iPad deals with weaker signals, severely reducing performance once a signal strength 'breaking point' is reached, 2) there is a hardware fault with my particular iPad, 3) the iPad 2 in general suffers from weaker signal strength in comparison to other products
It's a shame Apple removed wifi analysis apps from the app store last year as it would be interesting (and helpful) to be able to accurately determine what signal strength it is detecting in comparison to the iPhone / laptop / netbook etc.
Sorry if this doesn't answer too many questions but I thought I'd add my findings. I've got a hi gain wifi antenna coming next week which I'll try and post any results.