Slow Cooker

Someone who takes an ungodly amount of time to understand something
or, occasionally used to describe someone who is incapable of comprehending anything.

"I tried to teach my stupid Aunt Helen how to use her smart phone, but she is such a slow cooker."

"The sow cooker driving behind me had their high beams on. I had to flickered my car lights about twenty times before the slow cooker realized why that they left their high beams on.