I know it is never good to memorize opening lines and play them without understanding the plans and ideas, but still, I see so many grandmasters play moves from #5 to around #10 so slowly as if they've never seen the position before. But we know that they have, and furthermore the positions has been played in literally thousands of games by others. Are they trying to decide which of the moves that are in their repertoire to play? Or are they trying to fully absorb the plans, even though they've already studied the plans at home? Or are they trying to recall the variations that they studied at home but might have forgotten? What's going on in their heads during their slow play in the first 10 moves?
Remember that they've usually spent a few hours before the game to prepare against this particular opponent: reviewed which openings the opponent plays and what his weaknesses are, chose what kind of opening to play today, made a guess which variations are likely to occur and made a plan based on that, et cetera. He always plays the same against 1.e4 and I have a nice idea against that line, maybe play that today even though it's not my main move? In that other line, he recently won spectacularly against a strong opponent but I have an improvement early on, he probably hasn't looked at it too closely...
Of course, the opponent has done the same.
If everything goes exactly as expected, the moves go very fast. But usually at least one side goes for a surprise, and then the exact preparation is out of the window, and they have to rely on the general preparation. He suddenly goes 1.e4, maybe I should go for a secondary defense? But I know my main one so well... Or: he suddenly goes for the French! What have I played against this recently, should I avoid those lines? What other lines are there that he probably isn't that good at? Why is he playing this?
The moves and variations themselves are very familiar at that point, but there's still the metagame of choosing the right opening against this opponent.
Thinking long in the early phase can have psychological reasons. The grandmaster thinks how he can play against a particular player to exploit the opponents weaknesses, or tries to surprise the opponent somehow.
But in some cases, the opponent plays rare variants. Not all playable openings occur at grandmaster level and perhaps the opponent tried to come up with a surprise.
Even a grandmaster does not know EVERY sensible position possible after about 5 moves, only the most common ones, or those they play regular.