The word multitasking means different things to different people. To startup people, multitasking is a way of life. Being able to do more with less is what drives a startup towards progress. You have little in the way of money, time, and resources, to do what companies ten times larger than yours fail to do. Like a soccer mom on a mission, startup people rely on multitasking to get through the day efficiently, regardless of how long their to-do list might be.
I have read articles which discuss using "focused effort" to get through a startup. I don't know who these people are, but I have seen time and time again where focus kills an otherwise great startup. Focus is supposed to be a tool -- a means to and end where a person keeps their attention and efforts on a single task. This, in a startup, is career and company suicide. I have seen engineers who would spend an entire 3 hours creating a CAD drawing... just one drawing. In that time, I would have already created a sketch, sent it off to 3-5 vendors for quote, built a functional prototype to verify design characteristics, developed a test fixture to test that prototype, and had a cup of coffee and a Twizzler. That's multitasking.
I have both taught and demonstrated multitasking at numerous companies, large and small. The foundation of any good multitask effort lies in the fact that every action takes time. For every action that you take, there will ultimately be a period of time where you will have to rely on another person to react -- a vendor with a quote, an online order for tools that needs to be shipped, a test fixture that needs to cure before you can put water into it. These are the breakpoints where you need to switch gears, and it happens multiple times every day.
Focus seems to be the comfort zone of most people. The nice, calm, directed effort to complete a task seems very professional and very "office-like". Let's walk around the office with a cup of coffee in our hand, and chit-chat while we wait for a Solidworks update to download. This is office worker paradigm circa 1960. Today's worker is faced with shorter timelines and bigger goals. Multiply those timelines and goals five fold, like what you find in a startup, and you realize that a startup just can't survive with that type of work mentality.
Whether you like it or not, there are just not enough hours in a day to get done what needs to get done. Multitasking seems very chaotic to many, but if learned correctly and applied effectively, you can and will do the work of 2-3 people, the whole time feeling relaxed and satisfied that you're getting things done.
So un-focus yourself, and get those 15 tasks finished before lunchtime... your coffee and Twizzlers await you...