If you've upgraded to El Capitan and found your Mac laptop getting ridiculously hot after a short amount of time - EVERY time you use it - then here's how to help solve this (very serious) issue...
Diagnosing the issue
The first thing you need to do is open up the Activity Monitor Application (located in Applications/Utilities) and determine which processes are consuming the most energy cycles. Chances are that you will find the following processes (or a variation of) being the sole culprit:
These processes relate to the Spotlight functionality that's built into OS X and, for some inexplicable reason, manage to consume massive amounts of system resources every time your laptop is run. This problem appears to have started from the release of Mavericks OS X and continues to persist through to the El Capitan upgrade which is a less than ideal situation for everyday usage (particularly as we have personally experienced 2 logic board failures and subsequent replacements as a result of massive amounts of heat being generated when using our laptops - so you DO need to resolve this or it WILL damage your machine!)
It's not an ideal one but you need to kill all Spotlight processes in your Terminal application (located in Applications/Utilities) - caveat emptor: doing the following WILL render your Mac unsearchable in both the Finder and Mail applications so proceed with caution:
sudo mdutil -a -i off
The use of the sudo utility allows you to run in a root-like privileged capacity so you will be prompted for your password when running this command.
Once you execute this command and enter your password you should see output akin to the following:
2016-01-11 11:22:26.155 mdutil[1899:48783] mdutil disabling Spotlight: / -> kMDConfigSearchLevelFSSearchOnly Indexing disabled.
So far, so good!
In addition to the above it would be a wise idea to also delete the Spotlight database. Doing so allows us to clear any indexed entries from our Mac, freeing up space and preventing the re-indexing of certain directories/files (which will happen when we re-enable Spotlight later on after cleaning and optimising our Mac for this purpose).
To remove the Spotlight database simply run the following command in the Terminal:
sudo rm -rf /.Spotlight-V100/*
As a final measure we should also instruct Spotlight as to which directories/files we DON'T want indexed. This will help with reducing the amount of time and power consumption spent on the system processes allocated to Spotlight.
To do this, open up your System Preferences, click on the Spotlight icon and under the Privacy tab select and add the directories/files you do not want indexed.
Now the disabling of Spotlight is a less than ideal solution going forwards as it WILL render your Mac and Mail application unsearchable so you will need to re-enable Spotlight after making the above changes.
To do this open up your Terminal application and type out the following command:
sudo mdutil -a -i on
We would recommend that you perform this task and let Spotlight run overnight as, depending on the quantity of data being indexed on your machine, this can take a long time to complete.
Hopefully this helps with reducing the amount of heat generated by Spotlight's indexing and keeps your Mac laptop running a lot cooler!