Grain of salt, but I've been told that even on a 520 our old friends loading and resuming have been slain
Yes. And Microsoft has been trying to fix that in Windows 10Doubtful since they are still present on my 1520.
I know that but even with 8.1 the speed reduced greatly. I'm asking if 10 will fix it or not. The 521 isn't high end.Windows 10 is more like Windows Phone 8.2 --- So yes it should run smoothly.