Are you sure it's not just your events leaking objects? As in, the object count constantly increasing over time?
It's normal for UIDs to keep increasing, Construct just increments the global UID counter every time it creates a new instance, so the current UID is meaningless in terms of performance.
Unless you have specific measurements pointing at a specific cause like GC, it's a waste of time to start guessing about things like that. I'd first try C2's own profiler, but if that doesn't help pin it down you can try running a performance profile with Chrome's dev tools to see where most of the time is going. (Make sure you don't minify so the names are readable.) You can save and share profiles so I can have a look at anything you record and see where most of the time is spent in the engine.
Also modern mobile devices aren't really that special compared to desktops any more, if there's a real performance difference you should be able to reproduce it on desktop too, albeit with higher measurements (e.g. 60 FPS but a change in CPU usage or something like that).