Un-install Yosemite.
During the last Security Update the process installed Yosemite too. Now my old iMac is too slow to use. Can I un-install Yosemite? Go back to the previous OS X?
iMac, 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Nearly all performance-related concerns coinciding with recent OS X upgrades have been due to the presence of outdated, poorly supported, or ill-conceived software that should never have been installed to begin with.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.
