If your open rate is that low, you probably should do a very thorough list cleansing. You're probably sending to many, many people who haven't opened your email

If your open rate is that low, you probably should do a very thorough list cleansing. You're probably sending to many, many people who haven't opened your emails in years. You should stop sending to them, which will improve your metrics, in terms of spam filtering. I would also look at the open rates for new subscribers during their first month on the list -- these should be quite high. If not, then you have much deeper problems to address than the timing of when you send the emails.