Keep in mind that I don't do any work with fonts really, so it didn't matter to me if I lost any special fonts because I never actively installed any.just accumulated alot of extra fonts over the years from various apps and rolling my data over through multiple Macbook incarnations. It definitely seems to be a Font related problem, and since I could create a new user on my machine and Excel ran fine from that login, I did some detective work with fonts and worked out a way to strip down fonts in my regular account log in. Then Excel just began running incredibly slow as everyone here has experienced. I never had any problems with Excel until I got a new Macbook Pro over the holidays (Jan 2013) with Mountain Lion pre-installed.
