T-mobile seems to lag in general on updates. It appears they do this with Android phones as well.
I only went with the 521 because I needed a smart phone while waiting for the successors to the 1520 & 1020 to be released. No regrets with WP 8.1, just T-mobile service.
It is very similar to wp8 in performance. I did factory reset after update. Sliding keyboard is great improvement in day to day use.Does it lag for you? It does on my 521 sometimes, it gets annoying.
Also, how's cortana for you? Slow for me, but my gosh is she useful.
Pain in the neck to have to reinsert password to get wireless each time I wake the unit up.
Is this a know bug in the developer's trial??
Update:Nope, I have not heard of it until now. Have you tried a hard reset/factory reset?