As suggested in https://codereview.stackexchange.com/q/170958/14237, having a tiered cache would make common cases much faster than the several reads from disk that are currently required.
One potentially easy option would be to use
/tmp (but falling back to
/dev/smh is busted). Need to account for (and document?)
/dev/smh behavior better, since previously I'd seen files randomly disappearing from under me.