Q: OS X El Capitan is very slow like a snail

Same problem here with my iMac ... and I'm still working to fix it.

Go to http://www.etresoft.com/etrecheck — download and run EtreCheck, copying the status report to paste here.  It'll show us what's on your computer right now, and if there's anything on it causing all the slowness you're experiencing, we'll see it and advise you on where to go from there.

When you see a beachball cursor or the slowness is especially bad, note the exact time: hour, minute, second.  

These instructions must be carried out as an administrator. If you have only one user account, you are the administrator.

Launch the Console application in any one of the following ways:

☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)

☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.

☞ Open LaunchPad and start typing the name.

The title of the Console window should be All Messages. If it isn't, select

          SYSTEM LOG QUERIES ▹ All Messages

from the log list on the left. If you don't see that list, select

          View ▹ Show Log List

from the menu bar at the top of the screen.

Each message in the log begins with the date and time when it was entered. Scroll back to the time you noted above.

Select the messages entered from then until the end of the episode, or until they start to repeat, whichever comes first.

Copy the messages to the Clipboard by pressing the key combination command-C. Paste into a reply to this message by pressing command-V.

The log contains a vast amount of information, almost all of it useless for solving any particular problem. When posting a log extract, be selective. A few dozen lines are almost always more than enough.

Please don't indiscriminately dump thousands of lines from the log into this discussion.

Please don't post screenshots of log messages—post the text.

Some private information, such as your name, may appear in the log. Anonymize before posting.

When you post the log extract, you might see an error message on the web page: "You have included content in your post that is not permitted," or "The message contains invalid characters." That's a bug in the forum software. Please post the text on Pastebin, then post a link here to the page you created.

If you have an account on Pastebin, please don't select Private from the Paste Exposure menu on the page, because then no one but you will be able to see it.

what are your system specifications? What kind of mac is it and how much RAM does it have?'

Are you running any Mac AV or 3rd party product (clean my mac, mackeeper, etc.)?

If so remove them immediately as instructed by the developers directions.

Have you checked and verified your hard drive for errors using Disk Utility?

Using Disk Utility to verify or repair disks - Apple Support

Is FileVault enabled? If so disable it.

OS X Yosemite: Turn off FileVault encryption

Have you reset the NVRAM? If not do so now

How to Reset NVRAM on your Mac - Apple Support