I think you have a good point here. As a general rule, I would say it is better to invest your time in quality activities rather than just quantity. I think that when it comes to progressing with life goals:
"what is important is rarely urgent; what is urgent is really important."
-Stephen Covey.
I personally do not multitask. I take my time to plan out my week and then tackle one objective at a time. This reduces my stress so much and allows to give 100% of my mental energy to what I do at a given time.