Would suggest you to follow the below steps :
- Go to Lightroom Preferences > General > uncheck "show add photos screen"
- Lightroom Preferences > Performance > uncheck "use graphics processor"
- Restart Lightroom and check if its working better
Its both. Lightroom 2015.2.1 is extremely slow on El Capitan for lots of people. Not everybody but the problem is widespread and it is less of an issue on Yosemite (but not gone!). I can't run 2015.2.1 on any of my machines without going crazy regardless of how I set the preferences. If Chetna's changes don't help, you should probably revert to Lightroom 2015.1.1 as I did. Instructions here: http://www.lightroomqueen.com/how-do-i-roll-back-to-lightroom-2015-1-1-or-lightroom-6-1-1/ Those are somewhat easier to follow than Adobe's own instructions: Lightroom 6.2 Import Update.
Lightroom 6.3/CC 2015.3 is now available -- try updating to that version which includes the previous import functionality, bug fixes, and new camera/lens profile support: Lightroom CC 2015.3 / 6.3 now available
Adobe people have got to see this thread, after I upgraded my lightroom to the latest version, it became slower than a turtle, it's ridiculous, not fun and I hope they fix this thing with a new upgrade because I am thinking seriously of changing software.