The reason for email delays is commonly caused by not having enough emails to support the volume of your campaign or the drip rate being set too high to support the volume of your campaign.
Keep in mind that email campaigns are completed by using all emails that are available on your account. For example, if you have a email touch going out to 12 contacts and you have two email accounts set up, each email will send 6 of the emails at the same time. If the drip rate is set to 10 seconds per email. This campaign would complete in 60 seconds.
For the same example, if you had three emails set up, each email would send 4 emails. With the same drip rates this would take the campaign time down to 40 seconds.
This is a simple example. But, you can see how times will get to be significantly higher with campaigns to 500, 1000, or even more contacts.
While you may be able to cover the sending limits, you need to have enough emails to get the campaign out in a reasonable time.
Always make sure you have several emails set up and a drip rate between 3-10 seconds on your emails to ensure they go out quickly and there is no delay in your campaigns or the email delivery.