Ever since I installed Windows 10 builds on my Lumia 520 it's dead slow. New builds don't change that.
Working on Lumia 930, but it's draining my battery like crazy. The Battery-Drainage got introduced with the previous build and went up by maybe 50%.
I ditched mine. I'm pretty certain it has only 1/2 Gb of ram, Windows 10 mobile should have a minimum requirement of 1 Gb ram, makes a world of difference. Still small issues. I bet a little more would be perfect but runs fairly well with 1 Gb. Fairly certain that won't improve for phones with under 1 Gb ram. I upgraded to a 640XL with 1 Gb ram for that very same reason
Try hard reset (settings - system - about - "reset your phone"). It is like a clean install. Just updating might cause some troubles in some cases.