I read somewhere – can’t find the quote – something to the effect that Magnus already knows which moves won’t hurt his position, and the extra time is just him exploring difficult-to-see possibilities that might provide an added advantage.
Not using enough of your time is just as much a mismanagement as using too much of it. One player being “behind on time” could mean that they’re thinking too much, or it could meant that their opponent isn’t thinking enough.
I suppose you might get a clearer picture by comparing each player’s time, not to their opponent’s, but to a historical average of other games at that move number. Does this player, at move 20, have more or less time left than typical past players at move 20?
I’m thinking of a situation where one player is in serious time trouble and the other is not. The player who is not in time trouble would try to move quickly because that deprives his opponent of some calculations.
He’d probably also strive for moves that complicate the situation, so that a need to move quickly risks overlooking an important subtlety.
Actually I would say that the situation on the board matters more than the time situation.
Assume your opponent is very short of time.
If you are winning, then rushing yourself just means you might blunder - you don’t gain anything.
If the position is level (not completely drawn, just equal chances), then you are far better off using your time to calculate carefully rather than bashing out moves. (I remember one game where my opponent literally had seconds left for about 8 moves. N.B. this was a long time ago and we were using wind-up clocks. I decided to rush him - and my first move blundered a pawn! So I offered a draw and thankfully he took it.)
If you are losing, then fair enough if you want to rush.
But in all cases thoughtfully preparing a couple of moves ahead by using some of your extra time is a jolly good strategy.
I can add another game situation (from the British Championship 
)
My well-known opponent regularly got short of time - but clearly enjoyed the adrenalin rush and played pretty well at speed.
He had a couple of minutes for 10 moves; I had well over an hour.
So instead of ‘playing into his excitement’, I went for a walk (leaving my clock ticking.) After 20 minutes I returned … and he had calmed down. I played a few careful moves - and he lost on time.
As someone who:
- dabbled at chess (long before computer opponents were widely available )
- but realized it wasn’t my game, yet
- remains fascinated with the game and its elegance
- and follows the matches without understanding the nuances.
Your posts…humanizing the game, while still offering actual analysis, are among the (many) gems of the SDMB and are a part of why of I will always be a member of this board.
TLDR: @glee rocks!
Most kind. 
If you would like a practice game here against me (with analysis and ‘take back’ option) do ask.
Here’s one I made earlier (divided into the game itself and comments on the play:
Chess training game between glee and NAF1138 - Thread Games - Straight Dope Message Board