A study by McKinsey & Co found that workers spend up to 28% of their working week reading and answering emails. That's almost 1 out of every 3 work hours spent reading or writing emails. Exactly how much of that time is productive and how much is unproductive we don't know, but experience tells us that a good chunk of it is being wasted.
With email being the standard for business communication these days, people tend to default to email without even thinking about the time cost. Email is great for sending a message that someone can pick up at any time, and email is great when people need a written record, but email is terribly inefficient for discussion.
Think of how many questions can be answered in a two minute phone call. Two minutes of email time will barely get you past the introduction.
The other issue with email is that it is a constant interruption and distraction. If you read your emails every time a notification pops up, you disrupt your focus, and it can lead you onto other tasks away from what you were doing. When you finally get back to what you were originally doing, you have to reorient yourself with the task - and this wastes time.
Email is not all bad though; it is actually a very useful and convenient tool - you just need to manage it correctly to ensure it doesn't drain your time