I’ve read that the lack of sulfur compounds has that effect, and my subjective impression is consistent with that, but I haven’t done structured trials. (And I don’t prefer sweet onions in those applications, in any case.)
This makes a lot of sense and goes a long way to explain the time discrepancies. It's sad that I had to scroll down the page and find these comments buried at the bottom when they should be at the top.
Whatever kind of sweet onions we get locally start to turn brown after 7 minutes on medium high (keeping the skillet around 350F).