I don't know why previous versions of the app would load the sheet music faster, but I can give a little background about what the app's doing and why it might take some time. I'm a quality assurance engineer on the Church's mobile apps team, and I'm also responsible for maintaining the content channels for the LDS Music app.
The first time you open a song in a collection (for example, a song in the English hymnbook), it will download a combined PDF that includes all of the hymns for that collection. As it's downloading, you will see a "Downloading..." message on the screen. Downloading will only happen once for a given collection (so, once for the hymnbook), unless the PDF is updated, or you clear the app's data. The last time the PDFs were updated was several months ago ago.
What's most likely taking a long time is the "Loading..." process (not the downloading). Android doesn't have native support for PDF rendering, so when you go to a song, LDS Music has to convert that page of the PDF into a PNG that can be displayed. PNGs take up much more space than PDFs, which is why the rendering is done on-device instead of being downloaded as PNGs directly. The time it takes to convert the PDF to a PNG varies on different devices. After the PNG for a song is generated, it's stored in the Android cache so it can be pulled up quickly the second time you view the song. After the cache reaches a certain size, though, the older items in the cache get cleared out, so you may have to re-render a song if you haven't visited it for a while.
The developer (a Gospel Library developer working on LDS Music during his spare time as a volunteer) has been trying various methods (different third-party libraries, using SVGs, improving caching) to reduce the loading time, but unfortunately the options are pretty limited.
One thing we're prototyping right now is using MusicXML instead of PDFs to render the sheet music. If we get the approvals we need from the music department, this will really speed up the loading time – and bring with it several other benefits. We've wanted to use MusicXML for several years, in the Android and iOS apps, and I think we're getting closer to the right time for it to happen.
Samuel Bradshaw • If you desire to serve God, you are called to the work.