Hi,
What is the make and model number of the device?
Have you tried restarting it to check if the display is now OK?
Does your device meet the minimum system requirements to run roblox without problems - you said that lagging wasn't anything out of the ordinary?
If so have you checked what else may be running in the background (anti virus, other programs etc) that may be using system resources that if not used may help roblox to function more smoothly?
What have you checked?