Develop doesn't use the previews so it has to render each time.
If the images are in the Camera Raw cache already, or if they are DNGs with fast-load-data embedded, it will be somewhat faster.
I agree this is totally unacceptable and this was never so slow in 3.6. Overall, this program needs a performance boost all around and soon please!
I totally agree with you. I'm running Lr 4.1 RC2 on Win7Ent64 with i7-2700k, 16GB RAM and catalog and previews on a SSD drive ant all the Develop module is relly really slow and sluggish. I have 1:1 previews generated. Very very bad performances!!! Adobe wakes up!