It seems that Add Photos Screen added in the latest Lightroom is causing performance related issues.
Please follow the below steps to disable it.
> Go to Lightroom > Preferences.
> Click on the General tab
> Uncheck “Show ‘Add Photos’ Screen”
> Restart Lightroom
Let me know if this helps.
Wow yes it did, at least as far as I can tell so far. How did something seemingly so unrelated manage to screw with working in the Develop module? THANK YOU!
OK scratch that, after a while it seems that the performance has gone to crap again. It's like the longer you have the program open, the worse it gets...
LR CC 2015.2 / 6.2 and 2015.2.1 / 6.2.1 have many serious problems, so bad that Adobe has apologized: Lightroom 6.2 Release Update and Apology. Lightroom 6.2 Import Update. Rather than waste more time trying to troubleshoot the problems (with only a modest chance of success, based on the experiences of others reported here), I recommend you just revert back to 2015.1.1 / 6.1.1, the last stable release: http://www.lightroomqueen.com/how-do-i-roll-back-to-lightroom-2015-1-1-or-lightroom-6-1-1/