My iMac Retina 5k is running very slow after OS X El Capitan update.

After upgrading to El Capitan, my new mac is now very slow. Firefox, Safari, Chrome take about 20 seconds to load. MS Office 2016 takes about 1.5 mins to load.

I have about 500gb of hard drive space.

I don't have any cleaners or malware software.

My mac is about 3 months old brand new.