I find that my very large Aperture library takes a very long time to load. The "processing" indication runs for 10-15 minutes after launching. If you try to do work before it finishes processing, everything is slow and choppy, and prone to crashes. This was the final straw for me to make the jump to Lr.
I was very slow to warm to Lr, and worked with it for about 2 years before I decided to migrate my back catalog. What I learned is that Lr is much more powerful and versatile, and has better integration with Ps. This is to be expected, since Aperture has not been updated for so many years, and the technology has moved on without it.