This is one of the most annoying problems ever. Safari simply doesn't play videos smoothly unless mouse moves. I followed all the steps mentioned before (clear cache, delete files, history, cookies, etc...) but the issue is still there. Usually the problem disappears for a while if I do system reset but comes back later.
iMac with Retina 5K display, iOS 11.2.6, 27"
1. Turnoff or uninstall extensions and test.
Section: Manage extensions: https://support.apple.com/guide/safari/use-safari-extensions-sfri32508/mac
2. Delete com.apple.Safari.plist file and test.
Empty the Trash.
Quit all applications.
Hold the option key down and click the "Go" menu in the Finder menu bar.
Select "Library" from the drop down.
Library > Preferences > com.apple.Safari.plist
Right click on it and select "Move it to Trash".
Restart and relaunch Safari holding the shift key down.
If this doesn’t help, “Put Back” the com.apple.Safari.plist
Right click on the Trash icon in the Dock and select “Open”.
Right click on the com.apple.Safari.plist and select “Put Back”.
Select ”Replace” from the dialog when it appears.
3. Sometimes third party software can cause this kind of a problem.
Please run EtreCheck and post the report here.
Download it, open Downloads folder, click on it to open and select ”Open”.
Select a problem from the “Choose a problem” popup menu box, and then “Start EtreCheck” in the dialog.
Click “Share Report” button in the toolbar, select “Copy to Clipboard”, and then paste it when you reply.
Try setting up another admin user account to see if the same problem continues. Also try the Safe Mode.
Safe Mode - About El Capitan
also Sierra
Safe Mode Startup – El Capitan. also Sierra
Video freezes in Safari unless mouse moves but sound continues
