Right now I only store encrypted notes in nostrdb, so I might need an in-memory cache for decrypted stuff. The in-memory format for compact binary notes is the same as on disk, so you get the memory savings there as well. I still use this for damus ios and it made things a lot faster and use less memory.
Discussion
This is actually explicitly supported in the rust bindings, as you can see you can have an owned or db-owned note, where the db one is tied to the lifetime of a transaction. But in either case its just a pointer to memory or virtual memory
Simple apps should be able to handle decryption while scrolling just fine. So, I am not sure if anything more complicated than that is worth including in your lib. But more complicated UI components might trigger way too many pre-processing algorithms in parallel when scrolling and then pre-caching is the only viable answer. And storing this cache can be beneficial if the sum of all the pre-processing stuff to create those caches on the fly takes longer than loading them from the disk, which in many of our instances, already do.
Some time ago you mentioned that you were adding the .content parser to the db lib as well. That aligns with the cache needs here as well. Being able to pull already parsed .content from the db can provide significant gains in the rendering performance just because the CPU is left alone to do the other things the app might need.