Hi Pete,
I'm running the latest PI on an iMac with OS X 10.11.3, with no problems. A few things to check - how much free disk space do you have, and have you run the Disk Utility First Aid to see if there are any drive problems? Have you run any other software that will cause swapping to disk to see if there are any slowdowns there? Where is your /tmp folder located?
I hope this helps,
Sean