I actually loaded up your account on my device late last night, and right away (after about an hour and a half of syncing) I could tell what the issue is. You’ve reached epic status!
You have over 100k items, and 170 lists on your account, and ~48k due reviews. That is a lot of stuff, and the app is working hard to make the magic happen.
The database that Skritter stuff is stored in is kinda old by today’s standards (especially the way this iOS app is running things). So, any time you’re loading up lists like this the app is trying to sort through all your items and see what is due, and what isn’t. List size probably don’t matter much, but it is has to loop through the entire list items and then match it against your reviews so I would expect larger lists to take longer. Additionally, it has to talk to our server to do this, so you’re also in a queue with other people, which can also cause slower loading times if tons of people are studying at the same time. Also, since it is not calculated on the device, it can be prone to errors, which could be why you’re seeing the app not return anything sometimes.
Your device, although not terribly old, is also probably not helping things. Apple’s recent OS updates are mostly optimized for the new phones, and they can be taxing on older devices by themselves. While I didn’t do any studying on your account, I did load up a few lists on my iPhone X and it took a little bit of time to filter through things. Using the general study button did load something like 46k items, but it also took a while.
There isn’t really anything that we’re able to do in Skritter Chinese app to improve this. However, the good news is that I was able to load your account in seven minutes in the new app, and some quick tests of deck level testing appeared to be working okay. I know that you’re looking for mnemonics on study cards, and some other things to really make the experience better for the way your’e studying (and we’re working on some of that stuff), but I do think that you’re going to find that Skritter: Write Chinese will do a much better job handling an account of this size, if only for the fact that we’re using some much more modern ways to storing and sorting through data.
If you’re set on using Skritter Chinese app on iOS for the time being, I would recommend going into “Study” to force items to load into review. It should help, as it sounds like you noticed. I’d recommend doing that before you’re doing Advanced Study for the day, even if you don’t intend studying all items from all decks in one go.