Source: WowAce

Tooltip rendering takes an obscene amount of time for some users. After looking at the code, I believe it might be possible to reduce the performance impact, e.g. by using modified search and sorting algorithms, though extensive testing would have to be done.

Perhaps the way that the items are fetched from the DB could also be altered, e.g. a staggered or even dynamic (asynchronous) procedure to minimize the startup time (optional via settings. There could be other performance tweaks that would have to be enabled by the user).

Obviously the complexity has to be considered and fundamental changes to the addon's core shouldn't be made lightly.