I recently experienced very slow startup of my iMac and it seemed related to heavy Spotlight indexing. I researched this item on Google but I could not find a real Root cause.

I made a call to Apple support and I got the following suggestions:

  1. Goto to Finder
  2. Press Alt
  3. Goto Library
  4. Move folder Caches to the trash
  5. Move folder LaunchAgents to the trash
  6. Goto folder Preferences
  7. Move com.apple.spotlight.plist to the desktop
  8. restart

This resulted in my iMac performing as usual.

So it worked for me.

Be careful and make a backup before trying this fix.

No guarantees!