Does using an email for imessage cause message recieving delays?

On a 5th gen ipad messages are always arriving many minutes late, some don't arrive until you open the cover which unlocks the screen. I'm also wondering if using the screen cover that locks the screen is a separate issue.

I've updated it to the latest OS for this model, and reset network connections, tested it by texting it from an iphone on the same wifi and there was no difference.