Don't get me wrong. I totally agree that it's what happens in the time that is more important than the time taken itself. My problem is that with three different things going on at once, it's hard to reconcile them all into the same timeframe without one looking much the worse. It's a pretty problem to be sure. How to reconcile the time it takes for the Valar to create the Sun and the Moon, Fingolfin's host to cross the Helcaraxe, and Feanor's group to a) fight one battle, b) have Feanor killed, c) have Maedhros betrayed, and d) have Maedhros hanging out (sorry, couldn't resist) while, presumably, Maglor assume the role of leader (if not King) and tries to keep his brothers in check.
Maybe a way to solve this is to include the first contacts with the Sindar and Thingol by the Feanorians and have King Maedhros send his brothers out to 'scout' for new lands. This would set up some of the original tension between Thingol and the Sons of Feanor and would also explain how once Maedhros is rescued, he can send his brothers off to their timeouts because they've already established beachheads in those new lands.
I agree that the timeline must be compressed, but not every event needs to be
uniformly compressed. When we look at each event or plot point, we need to be asking,
"Does the duration of this event hold great significance in and of itself?" If the answer is no, then the duration of that event can be compressed. If the answer is yes, then the duration of that event should either be left as it was written in the text, or compressed only a little.
For example, I don't think the Feanorians need to take any longer than a month to reach Beleriand in the stolen ships, because A) nothing of note occurs on the ships, so the audience would likely lose interest if it took up a lot of screen time; and B) even though our story takes place in a world full of magic, taking 19 years to cross an ocean is going to be difficult for the audience to believe, for a lot of the reasons already mentioned in this thread (the fact that Columbus only took a year to reach the Bahamas from Spain, for instance).
The crossing of the Helcaraxe, on the other hand, should take
much longer by comparison (months? a year?), because a huge amount of character development would be taking place among Fingolfin's host during that period. By the time they emerge from the Helcaraxe, they are a changed people. They have endured hardships that the Feanorian host can't possibly imagine (yet), which will be a huge contributor to the tensions between the two encampments around Lake Mithrim.
Likewise, Meadhros' period of imprisonment should last a relatively long time, because the duration of his imprisonment holds a great deal of significance, both for him personally and for the Noldor in general. I don't think he has to be gone for
decades, but he at least has to be gone long enough that his return to Lake Mithrim is a BIG DEAL (for both emotional and political reasons).