can you provide a video of the behavior for folks to watch and see what you're experiencing?
are you saying that the game plays fine on phone at first but after awhile (how long is awhile?) it gradually starts to perform poorer and poorer?
phones have less ram / cpu power and overall resources than a PC.. it sounds like maybe something is spawning / being created over time creating more and more..
just consider maybe it's not the movement itself that's moving very slow.. but something else being created.
imagine something like stars being created for a background that are not being pooled / recycled.. over time every one of those stars if not destroyed / managed better will impact performance.