I've had Bitcoin-QT running for about six total hours, and the rate at which it's catching up is glacial at best. It takes about an hour to advance a week, which would make the process take about 4 days to complete. It didn't take nearly this long on my PC; I started and finished the process while my Mac was still running. I'm running Bitcoin-Qt v.0.10.1 64-bit on a 2011 MBP, 8 GB of RAM, on 10.10.4. Is this normal, or is there something wrong?
By the sounds of things and your fix, I would say that you had a blockchain created by an earlier version of Bitcoin-QT and something in the migration process in making that work with newer versions is causing it to take longer than to download the whole blockchain fresh with the latest versions. It's always advisable to keep a backup of wallet.dat (so you don't lose access to your wallet) but you can dump the rest of the application support directory which is mostly blockchain.