LR 4.1 RC: Creating 1:1 Previews becomes slower the higher the number of selected pictures

I selected 500 CR2 images and clicked create 1:1 Previews in Library module. At the beginning it took 7 sec for one preview. After 400 previews and 5 hous later (!) it took already 1,5 minutes for one preview!

I cancelled the activity. Restarted lightroom and the previews were generated swiftly again.

Used system: Windows 7 64 bit, 16 GByte RAM, I7 processor, SSD 120 GByte, two other fast harddisks. LR and Raw cache on SSD, catalog, previews and photos on harddisk.