Good morning,
Build 10149 runs great on the Lumia 520. There's a good bit of lag and some stuttering but it's nothing you can't just ignore. For the most part, Build 10149 runs just as well as Windows Phone 8.1 on the Lumia 520. If you can put up with graphical glitches, UI design quirks and a generally incomplete OS, and you know how to deal with potential bugs, go for it. Build 10149 is the smoothest build yet