Is there a way to specify that a certain app always stay in memory? I notice sometimes when I open the Reminders app (after not having opened it for a while), it takes a second to load. This is presumably because it is no longer in memory and loading from SSD. Is there a way I can tell OS X to never remove Reminders from memory?

P.S. I minimize Reminders, not close it. Which is why it sometimes stays in memory.
You can not set the application priority (to remain in the RAM). 

Your system automatically manages the resources based on current usage.

However, depending on your Reminders settings, and what external (Internet) accounts you use, it might take a moment for it to check those accounts.