People who think they are good at multitasking are actually bad at multitasking.
In fact, they simply tend to be the most distractable, and ironically slower at completing a group of tasks than someone who admits they are a poor multitasker and focuses on one task at a time. But the whole time because their attention is flicking back and forth between a bunch of things, have the self-evaluation that they're amazing.
http://www.pnas.org/content/106/37/15583.short
There's absolutely no reason to write down every hand or every hand you're involved in, and the time spent doing it is almost certainly better spent getting more hands played. Fundamentally, most hands played should be uninteresting - if you're not able to ignore 90% of hands as standard, then you probably need to revise your definition of standard.
Instead of writing down hands and trying to build a massive database of what your opponents have, you should simply mentally put them on a range and then count how often you're right. If you're right less than, say, 90% of the time, then expand your range, and if you're right more than 90% then narrow it.
Most of your opponents are not special flowers. They fall into one of four or five categories, and when you figure each of those out spend some time figuring out how to pwn a generic member of that category.
Then spend all your excess time shooting the breeze with your opponents so that they're playing their C game while distracted by your idiotic defense of the Patriots' blantant cheating or agreeing with your idiotic observation that that cocktail waitress is definitely not wearing underwear or your idiotic rationalization for shorting $WYNN stock, while you play your A game as a background task because everything is standard.