Currently we use a bounded mozGetAll search where a cursor search would be better in terms of perceived performance... With minor changes in the time controller we should show a substantial speedup to the first batch of busytimes displayed.
Does a bug like this one deserve a performance test?
I tested this while working on animations this week... it really does not work out as intended. Its still faster to use mozGetAll and then use setTimeout hacks to incrementally process the results... This is a perfect example of where having idb in a worker thread would be massively helpful... I will open up a new bug about improving the algorithm we use to load busytimes.