Q: 2013 Macbook is SLOW after El Capitan update.

      Reset NVRAM/PRAM.  http://support.apple.com/kb/PH18761

      Reset SMC. Choose the method for “On Mac notebooks with non-removable battery”.

      https://support.apple.com/en-us/HT201295

       Start up in Safe Mode. https://support.apple.com/kb/PH21875?locale=en_US

       If this doesn’t help, please run EtreCheck and post the report here.

      http://etresoft.com/etrecheck

      Download it, open Downloads folder, click on it to open, select”Open”  in the dialog to run it.

      Click “Share Report” button in the toolbar, select “Copy to Clipboard” and then paste it when you reply.

Activity Monitor - Mavericks  also Yosemite

Activity Monitor in Mavericks has significant changes

Performance Guide

Why is my computer slow

Why your Mac runs slower than it should

Slow Mac After Mavericks

Things you can do to resolve slowdowns  see post by Kappy

6 Easy Tips to Speed Up OS X Yosemite on Your Mac

OS X El Capitan: If your Mac runs slowly