Not CHDK related; but I though I'd share this in case anyone else is affected.
There is a pretty serious performance issue with Samsung 840 EVO SSD drives, that causes significant read slowdown as the data on the drive ages.
Samsung have release a tool to update the firmware and restore the drive performance (look for the performance restoration software section):http://www.samsung.com/global/business/semiconductor/minisite/SSD/us/html/support/downloads.html
It's Windows only at the moment and the drive has to be formatted as NTFS. The drive has to be direct connected to the PC (it won't detect in a USB enclosure).
There is a risk of data loss, so make sure you have a complete backup first.
Before the fix, my 1TB drive took over 5 hours to do a complete MD5 checksum of all 78000+ files.
After the fix the same operation took about 20 minutes.
It took a few hours to run the fix (not sure exactly how long as I gave up waiting and went to bed).