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Compared to previous versions of Mac OS X, El Capitan offers significantly improved performance and several changes to built-in apps, such as the ability to mute tab sound in Safari. It also makes Spotlight search both faster and more effective.
Compared to Yosemite, El Capitan uses smoother animations for opening or closing windows or navigating the Dock, among other actions. Though CPU usage is about the same, the end user will generally perceive better performance from El Capitan.
Compared to versions of Mac OS X released after El Capitan, such as Sierra, El Capitan generally is not faster. Mac computers dating from 2012 and later will generally run more smoothly when upgraded from El Capitan, per Apple's optimizations in Sierra and later. On the other hand, Macs from 2008 to 2010 may run faster with El Capitan.
El Capitan adds several upgrades to Mail. It now has a much more seamless full screen mode, offers several new mouse gestures, and it displays contact names and times more clearly in messages. See Mail itself while running El Capitan for a full overview of its new features.
Split View allows you to view two windows at once and have them occupy an even 50% of the screen. It supports multiple window configurations.
To activate Split View, simply click and hold the green, two-triangled button in the top left of the first window you want to Split View. This will move it into Split View. You can do the same way with any other windows you want to use with Split View.
El Capitan changes how windows in Mission Control are organized. To group windows by application in Mission Control as Mac OS X Yosemite did, go to System Preferences, then Mission Control, and select "Group windows by application".
El Capitan allows you to make the mouse cursor very large and visible, in case you lose track of it. Simply shake the mouse rapidly to do so.
Spotlight's indexing is much improved in El Capitan compared to Yosemite and earlier. Despite this, a common issue when upgraading to El Capitan is that Spotlight indexing does not work correctly. Often, this can be solved by going into System Preferences, then Spotlight, and unchecking "Allow Spotlight Suggestions ..." near the bottom. After two minutes, check the box again.
If you find it necessary to downgrade from El Capitan at any point, or revert an upgrade to Sierra, you can hold Command + R while restarting your Mac. You should be given the option to restore from a Time Machine backup, if one is available.