Question: imac very slow how to speed up?
Answer: Find resource-hungry processes. Some apps are more power-hungry than others and can slow your Mac to a crawl. ... 
Manage your startup items. ... 
Turn off visual effects. ... 
Delete browser add-ons. ... 
Reindex Spotlight. ... 
Reduce Desktop clutter. ... 
Empty the caches. ... 
Uninstall unused apps.