Adobe just sucks. flash, photoshop and lightroom are dogs even on 1 core. I've recently tried Bibblepro v5 on my older 8-core linux box (2GHz Harpertowns) and it flew through 21MP raw files and scaled well all the way to 8x.
I am really considering the switch from Lightroom to BibblePro. Unfortunately, there really isn't any Photoshop equivalent in terms of features..
Adobe. BARF!
Can it be totally ruled out that this isn't a problem because it's a hacked system - does the same happen with a "real" 12 core?