Hi ChrisR,
How much free space is left on the drive that the images are located on? If it is an SSD, those can slow down when they are close to full.
Otherwise, you can email us a copy of Photo Mechanic's log file to support@camerabits.com so we can take a look at it.
You can access the PM_log.zip file by going to Photo Mechanic's top Help menu and selecting "Reveal Support Data”.
Select "Yes" in the next dialog to create the zip file and then a window will open with the "PM_log.zip" file selected. Drag that to your desktop and then send us the file as an email attachment.