I'm not sure if this has been throughly discussed, but i'll bring it up anyway.
I often don't rush to get magnetism or navigation during the mideval era, i'm too focused on the early wonders and new weaponry that comes around.
but then i think, i'm leaving behind the technologies that let you trade over ocean squares.
imagine if you (China) were the only one with magnetism/navigation for, say, 60 turns. Say your buddy (Rome) has some extra silk, and he's looking to sell, and your other friend (Persia) desperately needs luxuries to keep his people happy.
The two may not be able to trade directly, so Rome trades to you, and you trade to Persia, at a markup of course.
Technically, if Rome had ALL the silk in the world, he would trsade you one, you'd trade it to someone (leaving you with none), and then rome could trade you MORE silk, and the cycle repeats.
the person that dives for magnetism may have a very lucrative late mideval era.
or, he may be crushed by those with the weapons to do it
