I'm still on windows 10 , i like it so much, but yes, is slow, battery performance is weak, but the build is more usable than the previous builds . I hope Microsoft will optimize windows 10 mobile for oldest phone with dual core processors like 520 , 620 etc . On my girlfriend 640 , windows 10 is running so good, it's fast, i don't know about the battery life, but i haven't heard her complain about it , neither about performance. For now, i will use windows 10 on my 620, hoping that the future builds will be much more improved.