You should really optimize how you save the email sending queue in the table _mailpoet_sending_queue.
On my websites, for the bigger one, this table is several Go and continues to increase with time... until the server crash.
Why are you saving each email in plain HTML in the database? You should save only one version of the global email and then optimize to call this email for each user... not save each email in the table, I have dozen of thousands of the same emails in my table...
Really, they're a lot of improvements to be done here! It's ok for small websites but for bigger ones it's not really possible.