I have two Macs -
I have Creative Cloud installed on both with the same version of Lightroom CC. I have my LR catalog stored in Dropbox, which is synced to both computers.
Here's the problem: on my Macbook Pro, my LR Library opens in around 15 seconds. On my iMac, the very same library takes over 3 minutes to open. I understand that my iMac hard drive and processor are slower than the Macbook Pro, but I wouldn't expect the difference to be so dramatic. I believe that it did not used to be this way, and that something may have happened on my iMac to slow Lightroom down so much. I have done a fresh install of Dropbox, but that has not fixed the problem.
I think it has something to do with storing your catalog file on dropbox. Try moving it to a local folder that is not linked to dropbox and test.