About 1.5 years ago, I tried to make an audiobook player for folders that displayed a cover.jpg and displayed the folder/file name – the thing was, just reading the folder contents and durations after setting it to play a big folder was so slow, the app became unresponsive and SFOS asked to close it a bunch of times. Because this was unbearable (and I did not have a clue about how to index folders performantly/in the background, but the app concept required some information about what the complete duration was), the app was discontinued before release. Recently, I ripped out the part of it that worked best (the sleep timer) and released it without the player.

Long story short: I feel your pain.