miners couldn't keep up with the blockchain difficulty
Such thing cannot happen. Difficulty depend on hash power. If hash power decrease, difficulty will decrease as well.
So when difficulty decreases, block rewards increase again? And miners are the ones handling the transactions in the bitcoin network, right? So when hash power decreases, transactions takes longer to get confirmed?
I was wondering what would happen if miners couldn't keep up with the blockchain difficulty.. Does that mean transactions will take longer to get confirmed?

Please learn what difficulty is before wondering about inexistent things
https://en.bitcoin.it/wiki/DifficultyPlease learn that not all people could understand what's written on that page.