I would be pleased if someone might reproduce my steps and report his/her performance issues.
Took a PV 2010 image and created a virtual copy.
Settings: No lens correction, linear tone curve, noise reduction to zero. No brushes.
Converted it to PV 2010 and "played" only with the colour temperature slider in both PVs: No remarkable difference, follows quite pleasing.
Manipulated tone curve and "played" with colour temperature as above: No remarkable difference, follows quite pleasing.
Tried one of Jeff Schewe's "Don't do it or it will slow down" settings: Added lens correction and doing the same as above: No remarkable difference, still pleasing.
Added luminance noise reduction (about 35): Now the picture changes (sorry for the pun)! PV 2010 slows down a bit, but now there is a remarkable gap in PV 2012! Set noise reduction to zero and performance levels were quite equal again.
Intel mainboard DZ68DB, i5-2500k (no overclocking), 16 GB RAM
Graphic: Internal HD 3000. Monitor: 1280x1024 + 2560x1440. LR running on the small display, single display configuration, switched big one on and off.
Windows 7 Ultimate, 64 bit.
Image by Canon 7D, RAW in about 23 MByte.