That's because you've got very few files, relatively speaking, on the cards.
Try it with 10-20,000 files ... browsing is slow and indexing is glacial. And notably so, compared to the SP2000 (which is much less powerful).
With Android based daps (it's not just A&K, just the SP3000 is currently a step backwards in this regard), using multiple, large, microSD cards, with high-file counts, is a very poor experience. I don't want to wait 10-15 minutes upon changing cards to have the player be usable again, much less have to manage multiple cards.
I'm sure the issue is with Android and it's storage card handling ... as I've written code to index music libraries that, running against the same microSD cards mounted on non-Android devices, completes the job in seconds, not 10+ minutes.