I noticed that my Lumia 535s got really slow after I have upgraded them to win10, a couple of months ago. Is it possible to downgrade to the 8.1? Should I?
Yes you can downgrade by Windows device recovery tool. But I recommend you to have the latest firmware and then perform a hard reset. It solved many problems for me as I am doing very well now with my MSL535 running on W10M. It is even more fluent than WP8.1. If you don't like that you can easily downgrade by Windows device recovery tool available at official MS site.