Please try these steps :
Click Edit-> Preferences -> File Handling
Purge both video and Camera raw cache.
Increase the size of only Camera raw cache to 30 GB.
Close Lightroom and re-open it
Now check the develop module.
I did that but nothing has changed (I regularly purge the caches) ...
Go to Help > System Info and check which exact version of LR you are running. If you are on CC 2015.2 / 6.2 or CC 2015.2.1 / 6.2.1, many of us recommend that you revert to CC 2015.1.1 / 6.1.1 as described here: How do I roll back to Lightroom 2015.1.1 or Lightroom 6.1.1? (Don't rely on your memory about which version you think you are running -- the Adobe update process is unreliable.)
Bad luck, this problem is with that 6.1.1 version I am running.