Hi to the 3d.
Last night, after indexing success in my MacBook Pro, I performed all the same procedures for deleting /var/folders/*, reboot and re-index my iMac.
The first time I launched indexing (as I wrote in one of my previous post, before Christopher discovered), this toke 3 full days, but it was successfully completed.
This time it was faster, 15 hours, but not as fast as MacBook (half an hour with a lot more of data!!).
I think it depends because in iMac I've 2 users profiles, in fact process monitor showed that there were 3 "mdworker" process (and 4 mds, one more for root):
1 for my user (admin)
1 for other user (logically...)
1 for _spotlight
The 3 processes often slept (no CPU activity) and still before the end (95% status bar 1 hour to left), there were 2 mdworker process under my user, and this I think is strange... (one was hang up?)
Also for me the problem will be find by Apple in Lion 107.5 because in the previous patch I've rebuilt entire spotlight DB more than one time, but I don't remember about slow indexing or TM Backup problems.
Does anyone have notice about this problem in Mountain Lion?