- skip updating buried cards on startup; it's expensive and we'll do that on
deck close in the future
- add an index for groupId. Initial profiling indicates that groupId-based
selective study is considerably faster in certain scenarios
The 50k element deck I'm testing with now opens and builds the queue in 40ms
on a cold cache, of which 34ms is the initial deck startup and 6ms the queue
build. Adding back the undo log and backups will of course increase this, but
this is a big improvement for checking due times in the deck browser.
