One time, I had a job interview where the guy asked questions from a stock list asked of all applicants. One of the questions asked what would I do if I had three or four things to do and needed to be completed at the same time. I answered that I would prioritize and do the most pressing thing first and work my way to the next important, etc. This was the wrong answer.
The right answer was that I would do them all at the same time. Looking back, I realize that the right answer was also the stupid answer. But that is Corporate America for you.
I have been multitasking for a long time, and I can tell you that it doesn't work. Invariably, when I do two things at a time, I do both so badly that I usually end up doing it all over again. I have also found that if one item takes 10 minutes to do and another takes 10 minutes by combining them I end up spending 30 minutes on them total. This is not time saved. It is time wasted.
Another bad habit is talking on the cellphone while driving. I either have issues with traffic, or I barely hear the conversation. Because I wouldn't take the time to actually pull over and talk to someone, I have lost information or gotten the wrong information. Whatever time I thought I was saving I wasn't.
The bottom line is that you can only concentrate on one thing at a time. That is why it is called "concentration." It reminds me of the shitheaded exec who told his employees that there were 25 things he wanted them to focus on. Needless to say, they weren't able to focus.
Do one thing at a time, and do it well. You will improve quality and save time.