My situation was in installing an aftermarket battery (I inherited a MacBook with too many cycles). After a few minutes in the GUI the MacBook would slow down to a crawl. I figured out, a few software versions ago, that I needed to delete the macbookpro8_2.plist file from /system/library/extensions/ioplatformpluginfamily.kext/contents/plugins/acpi_sm c_platformplugin.kext/contents/resources
This was easy to do in mountain lion and Yosemite, but not so easy when they introduced "rootless" to El Capitan. In order to delete system protected files, I had to go into the recovery terminal and run csrutil disable. Not having the menu to choose terminal from was a major upset for me as the GUI lag from the aftermarket battery made my MacBook absolutely unusable. It was literally click...wait five seconds for results for everything.
I was able to fix it by creating a bootable install USB (using my second Mac from work), boot my home mac to the bootable USB, disable rootless using the recovery terminal, boot to the GUI and delete the plist file, boot back to the USB, enable rootless, and my MacBook functions like new again.
What an effin headache :-/