If he had to look uptime through a potential timeline, he then snaps back to the present, to then look uptime through another timeline. In real time, that would take months, to years to check out each potential timeline, multiplied by more than a million timelines.
The time stone is keeping him young, when he should be several million months older, unless he was casting his astral form into the future, which would not age.
The time stone has fortifying his mind, to keep track of what happened in those million timelines, unless he didn't need to bother to remember all the failures. Just keep ticking through the losses to Thanos until they strike a victory.
But if the time stone makes you think that everything you have ever did is always happening now, that means that Stephen doesn't have to remember anything, because he's always reading every spell he has read and every spell he will ever read.