You cannot be sure how quickly your colleagues will respond to messages when you are absent, so avoid promising any instant assistance. Moreover, ensure you take permission from your co-workers before giving their details in the message. They might be too busy to substitute.
I might sound nitpicky but the language is important. “Might” or “may be” or “slower than usual” are vague and don’t offer the sender all that much information about when you’re really going to respond to them. Worse, they do a horrible job of protecting the time of the email receiver who, as the responder notes, is not in the office! Such a responder implies that, not only will the vacationer reply to the email, but they may not even miss a beat. They may be slow to respond, but they also might not.

For those new to the business world, your out-of-office message is the most common form of automation related to email. Once activated, it sends out a predetermined email message to anyone who emails you while you’re out, telling recipients exactly what they need to know.
I know a lot of people who never vacation for more than a few days because the email backlog becomes incredibly stressful upon their return; this is one way to meaningfully combat that which is somewhat in the employee’s control.