Separate relief layer?

Hello, as for being limited of phone storage I`m continuously looking for ways to reduce overall maps sizes. Because most of maps I use already contain hard-rendered relief shading this obviously decreases the map compressibility. Without wanting trade off the relief shading I wonder if removing the relief from all maps and rendering it from another source would bring a space saving without a significant performance loss.

There`s 2 ways how to realize this: either as a standalone relief only map displayed always on top as opaque layer or fully rendered from hgt files (if present) like in OruxMaps. The latter method has it`s clear advantage in fully eliminating the relief from saved maps thus size reduction but might have an impact on performance.

I`m curious how hard is to implement one of the ways and how demanding on CPU utilization/battery life would it be.