In reply to Mark.

Essentially all "fixed" positioning (incl. position: fixed + z-index overlay) have this "stuttering" / lagging issue on mobile / tablet browsers.

IMHO transforms look like a decent work-around - anything else Skip that - acc. to caniuse.com all current mobile / tablet browsers seem to properly support the position: fixed CSS property. So my suggestion would be position:fixed + z-index overlay (and maybe some out-of-viewport JS magic).

cu, w0lf.