So, what are your readers looking at during that 15-20 seconds? And, how much are they seeing during that brief time span? Sometimes readers are excited about the content and click on a link immediately, but sometimes they close the email very quickly.
Ultimately, this means that you have little time to grab your readers' attention. So, if your email has graphics aside from text, your readers are most likely spending some of that 15-20 seconds looking at the graphics, like your logo, instead of reading the copy you've worked so hard to write.
Your readers are only giving you a few seconds to convince them to take action and are reading a lot less of the copy in the email.