When you try to do too many things at the same time you usually end up not completing things as well as you could have. This is especially true with important work where you require concentration and the highest quality. If you make mistakes, you end up having to do things again, thereby duplicating effort.
Systematic execution in order of priority, with total focus on the task at hand, leads to a better outcome. Therefore, slow down, articulate your priorities and execute one thing at a time with total focus!