Only tested on Windows Store builds so far, but the Slider works great when using the mouse, but it is very laggy when dragging with finger/touch input. I thought maybe it was just a bad implementation so I rolled my own using OnPointerDown and OnDrag events. I haven’t debugged a huge amount, but I’m pretty sure the root issue with both my custom implementation and the built-in one is that OnDrag (and maybe other events) are not fired frequently enough.
Is anyone else seeing this, or am I alone on this issue? I haven’t seen any talk about it, which makes me think maybe it is specific to Windows Store (or Windows Surface tablets, what I’m using for testing)… otherwise lots of people would be talking about it.
Prior to the new UI system I made my own UI with Sprites and did all of my own input. Don’t really want to go back to that, but I guess I might have to if I can’t resolve this issue. I find it beyond annoying when games have UIs that don’t work well – maybe because I’m a UI developer by day.
I had same kind of problems on mobile device and it was pure performance issue that the slider had to be separated to be individually under separate canvas than any other UI element. Problem was that the slider makes everything under its canvas to be updated when being dragged. Maybe on editor your computer performance is OK and on mobile it collapses?
Edit: Now reading again I suppose you are testing the touch and mouse on same device. So nevermind 
Actually this is very much possible and I’ll check this evening. It is the same build, but I’m doing the mouse testing on my PC and the touch testing on my Windows tablet. I’ll try connecting a USB mouse to my tablet. If I’m having the same issue as you, I would think it would be laggy with the mouse as well, on the tablet.
Tested with mouse on tablet, and works great, and FPS steady at 60, so don’t think it’s actual lag… I think something in the Unity code is purposely (or bug) adding a delay. Possibly they are using a polling method, checking input info every 500ms to 1 second. It is only a UI slider issue though, because Input.Touch for general usage doesn’t have this issue.