Safari super slow when I'm on youtube

Please install once again , adobe flash player and drag drop in application folder , after that go to download folder , select the .dmg file & trash it , restart the computer & empty the trash .

Secondly , open safari preferences & select security and check the box internet plugins . make sure in plugin settings adobe flash player should be allow .Flash player is just supportive to run videos smoothly .

Also suggested article : Adobe Flash Player updates available for OS X on June 20, 2016 - Apple Support

i am not aware much of it . i also use you tube and check the updates .

Also some safari issues can occur if anti virus is installed , it is recommended to download apps from app store & avoid installing from google .

You have also written in your original post that you are deleting cookies that's so good .

Please verify that any extensions are installed , that must be deleted . But you are using El capitan so by default extensions is off .

Try this hit & trial method : click on go to go folder & type ~/library

From here see folders caches , cookies , saved application state , please select all contents inside the folders and right click on them to move in trash . Restart the computer and empty the trash .

Also all clear history & website data .

Please post if issue still persists .

Try the above first.

Try a restart.

Do a backup, using either Time Machine or a cloning program, to ensure files/data can be recovered. Two backups are better than one.

Try setting up another admin user account to see if the same problem continues. If Back-to-My Mac is selected in System Preferenes, the Guest account will not work. The intent is to see if it is specific to one account or a system wide problem. This account can be deleted later.

Isolating an issue by using another user account

If the problem is still there, try booting into the Safe Mode using your normal account. Disconnect all peripherals except those needed for the test. Shut down the computer and then power it back up after waiting 10 seconds. Immediately after hearing the startup chime, hold down the shift key and continue to hold it until the gray Apple icon and a progress bar appear. The boot up is significantly slower than normal. This will reset some caches, forces a directory check, and disables all startup and login items, among other things. When you reboot normally, the initial reboot may be slower than normal. If the system operates normally, there may be 3rd party applications which are causing a problem. Try deleting/disabling the third party applications after a restart by using the application un-installer. For each disable/delete, you will need to restart if you don't do them all at once.

Safe Mode - About El Capitan

Safe Mode Startup – El Capitan.

If it works in the Safe Mode, try running this program when booted normally and then copy and paste the output in a reply. The program was created by Etresoft, a frequent contributor. Please use copy and paste as screen shots can be hard to read. On the screen with Options, please open Options and check the bottom 2 boxes before running. Click “Share Report” button in the toolbar, select “Copy to Clipboard” and then paste into a reply. This will show what is running on your computer. No personal information is shown.

Etrecheck – System Information