Constant beachball and freezes without high CPU usage, "nspace-handler-update: did not find token" message We have a 2015 MacBook Pro here with OS X 10.11, which gets extremely (unusably) slow when starting a Time Machine backup.  All applications freeze from time to time and the spinning beachball keeps coming up.  The backup only goes to a few kilobytes and doesn't finish even after several hours.

During this time the CPU usage is not high, and the memory is not full.

What could be the cause and how can we fix this?

Other symptoms:  

nspace-handler-set-snapshot-time: and nspace-handler-unblock: did not find token messages keep coming up in Console.app.  After searching for these I found many complaints about the same symptoms online, but I found no solution.  This is one example of a forum thread discussing these problems.  The answers there suggest checking that there is enough free disk space and running a disk check.  Yes, there is > 400 GB free space and the disk check finds no errors.
Finally this gave the clue.

It was caused by Sophos Antivirus.  Uninstalling it fixed the problem.