Mass flow through the house and fan should be constant.
Mass flow through sliding door = mass flow through exit of room = mass flow through fan = mass flow out of attic
If the sliding door is closed to 2/3 the velocity will increase through the sliding door, but the mass flow should still be about the same. Is the exit of the room directly inline with the sliding door? Is it possible you're feeling more velocity at face level, and the areas near your feet have a slower velocity so the mass flow is still the same? You've got me curious about my house fan, I typically have a handful of windows open. Tonight I'll try reducing them and see if I can feel an increase in flow.