Under /data, restic saves pure data packfiles, but also tree packfiles which contain the metadata of all trees/directories.
Tree packfiles are accessed quite a lot, but they are cached. This, however, means that you have to download them (to cache) if you loose your cache or if you backup from different machines to a single repository.
Also, make sure that the cache is really used and don’t use the --no-cache option!
About prune, there is already the --repack-cacheable-only option which prevents prune from repacking data packfiles.
That said, I’d like to advertise rustic again. There, support for cold storage is already implemented (including multiple access to a repository / having --repack-cacheable=true as default for hot/cold repos / added possibility to allow prune to repack pack files only after they reach a given age).