Missing objects

Those can be caused by a lot of things, I guess. So it's kinda hard to know. First of all, for the disappearing sprites, that could be caused by clipping. If the Z values of the sprites are too close, they will clip and behave weirdly. So check that out, if altering the Z values help with that problem. And for the speed changes overtime, try moving the movement to FixedUpdate() rather than Update(), and use Time.fixedDeltaTime for calculating speeds and such. Not sure if that will work, but it's worth a try, I guess. Are you moving your character with ApplyForce()? I'm moving my character with that method, and I'm having the same problem as you are, but I'm working on 3D enviroment. Now for the walking left and going under layers, that could also be caused by moving with ApplyForce(), because if you apply one wrong force in one frame, your character could move in Z axis, and that would just mess up the whole thing. And again, if the new room you created is in a different height, your character will disappear when walking into it. TL;DR 1- Sprites could be disappearing randomly because of changes in the Z-axis 2- Try moving your movement to FixedUpdate() rather than Update(), and use Time.fixedDeltaTIme in your calculations. That COULD fix the movement speeds 3- The room problem could also be caused by changes in Z-axis, so check that out

hope that helps, if not just ask

unity does not need to warm up, the discrepancy in speed is probably caused by lag or other things taking up memory, what you need to do is tie speed into time, so you would do speed*Time.deltaTime or Time.fixedDeltaTIme if in fixed update, when applying speed in your code.