I also have this issue on both my old phone and my new one (pixel 7a). It seems to be related to loading thumbnails. Maybe it hangs due to the instance taking too long to respond.

I have discovered a sort of workaround: when it hangs press home then open Thunder again. It will be all black but will re-render when it comes back, so you can instantly tell when it is no longer frozen. Note that navigation still goes through, so swiping while the app is frozen will scroll the feed and maybe even vote on posts