Among his many tips and strategies, time management guru David Allen offers a "two-minute rule." He says that if you can get a new task done within two minutes, do it. If you cannot get it done that fast, either delegate it or defer it. In his words, deferring it means putting it "into your organization system as an option for work to do later."
Allen's books are bestsellers. People pay attention to him, and we writers should too. If your reader cannot handle your message or document easily within two minutes, it is going to be put somewhere "as an option for work to do later." Or ignored. Or deleted.
When it comes to email, people follow a 30-second rule before the two-minute rule. If they cannot understand what you want within 30 seconds, they go on to the next message. They tell themselves they will get back to your long or confusing message later, but often they don't get back--and you don't get action or a response.
Test your message. Can readers recognize in just a few seconds what you want? Can they complete the action within two minutes? If not, your message must be important enough to your readers to put on their to-do list to accomplish later.
I hope you were able to read this post within 30 seconds. That was my intention.