Ok, I posted a video here to try to show it. You can’t see when I tap the table of contents or settings buttons, unfortunately. Today, the table of contents is working normally. It slows down a little after I go into the settings menu, but not much. You can really see what I mean when in settings and I hit the back arrow and it highlights and takes a longer than normal time to go back.
https://imgur.com/a/mRbHluh
To answer your questions:
It does it on the 4 epubs I tried, and it is persistent even after force closing the app. I have no annotations or highlights in any books, so it isn’t that.
1. Is Low Power Mode on? - no
2. Does it happen in every book, or only some? - it does it on the 4 I tried
3. Is it slow right after you open a book, or does it get slower the longer you read? - no, it seems slow right from the start
4. Is the rest of JustRead slow too (Home, Library, App Settings), or only inside a book? - ive only noticed it in the toolbar, nowhere else so far.
5. Which reading theme do you use (and is your iPhone set to Light or Dark mode)? - day reading theme, app theme is set to default and I’m on light mode.
6. Do you use Read Aloud with a Natural voice, or Background Listening? Is Page Curl on? Is Per-Book Preferences on? About how many books are in your library? - not using read aloud, curl is off, I don’t use per book preferences, and there are currently 14 books in my library.
7. Do you use any accessibility features, such as VoiceOver, Voice Control, Switch Control, Bold Text or a larger text size? - no
8. How much free storage does your iPhone have? - ~69GB
Edit to add: I just hard restarted the phone and that did not change the behavior.