I don't know if I'm right, but I think there are two things to take into account. The flight model, and the graphics. Wings of Prey has an arcade flight model, but basic, though efficient, graphics. The contrary goes for Cliffs of Dover, it has a relatively good flight model, but relatively bad graphics, and runs like crap.
The priority should always be the flight model, then how well the thing runs and then graphics. Being a simulator or a semi-arcade game shouldn't have anything to do with how well it runs, I was playing some nice simulators 15 years ago on a 486...
So yeah, they really messed up with cliffs of dover, it was terrible at release, and even though all those patches fixed a lot of things, it's still not as enjoyable as 1946. So now I'm careful.
edit: This discussion makes me want to play euro fighter 2000 and a-10 cuba again!