Q: Un-install Yosemite.

If you created a Time Machine backup, use it: Use Time Machine to back up or restore your Mac - Apple Support.

If you did not create a Time Machine backup, do that now.

The only practicable way to "undo" a OS X upgrade is to restore your Mac from a backup created prior to upgrading. Otherwise, your options are limited to erasing the Mac completely, followed by re-installing a previous version of OS X such as its originally installed one, followed by upgrading to the OS X version you would prefer to use (assuming that's a later version), and finally migrating your content from a backup.

Having said that, you would be better off fixing what's wrong with your Mac: describe what's slow, and when it's slow. Find out if the same performance degradation occurs while running in "Safe Mode": Try safe mode if your Mac doesn't finish starting up - Apple Support.

Nearly all performance-related concerns coinciding with recent OS X upgrades have been due to the presence of outdated, poorly supported, or ill-conceived software that should never have been installed to begin with. There is almost never any justification for retaining a superseded operating system.

Try running this program 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.
  

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