Unfortunately EXCEPT is almost never what you actually want. It's a set operator, like UNION, and just like UNION it also has the side effect of removing duplicates from the result set unless you explicitly say EXCEPT ALL.
Because it's a set operator and not a join, it's usually very hard for the query planner to optimize it beyond the most trivial cases. It usually ends up being one of the last steps in the query plan. In a good query plan you almost always want to eliminate rows you don't care about as early as possible so you don't have to drag them along in every join operation only to have the data discarded at the end, but if you're using EXCEPT instead of the explicit anti-semi-join operator (that is NOT EXISTS(<subquery>)), you're making that very difficult for the query planner.
EXCEPT and INTERSECT are sometimes a handy shortcut when you're writing some quick and dirty query by hand, but I have literally never used either of them in a production query. You almost always want to use EXISTS() and NOT EXISTS(). They explicitly communicate intent, which is appreciated both by people reading the query and by the query planner, and they lack the footguns some of the other alternatives have.
Because it's a set operator and not a join, it's usually very hard for the query planner to optimize it beyond the most trivial cases. It usually ends up being one of the last steps in the query plan. In a good query plan you almost always want to eliminate rows you don't care about as early as possible so you don't have to drag them along in every join operation only to have the data discarded at the end, but if you're using EXCEPT instead of the explicit anti-semi-join operator (that is NOT EXISTS(<subquery>)), you're making that very difficult for the query planner.
EXCEPT and INTERSECT are sometimes a handy shortcut when you're writing some quick and dirty query by hand, but I have literally never used either of them in a production query. You almost always want to use EXISTS() and NOT EXISTS(). They explicitly communicate intent, which is appreciated both by people reading the query and by the query planner, and they lack the footguns some of the other alternatives have.