Sprites aren't made to work with 20-50 1080p images as frames, that's part of your problem.
A sprites job is to load everything into memory when it's created. So instead of loading one image after another the system hangs while everything loads.
While loading by url alleviates that problem by giving you asynchronous loading, it does not reduce gpu memory usage.
Memory is memory, it can't be reduced by sideloading it. In fact I think it can raise memory use as those textures may be ignored by garbage collection.
A better solution would be 20-50 sprite objects loaded one at a time from another layout via a timer so as to not bog down the system.