For about the 10th time, this is not a bug.
The Spitfire I has a 2 position prop. Fine pitch for takeoff & coarse pitch for basically everything else.
If you don't have CEM switched on then Santa's little helper is just putting the prop into coarse pitch for you after takeoff. If you land the aeroplane, you'll see that the prop is put back into fine pitch as soon as you've got weight on the wheels again, just in case you wanted to touch & go.
If you fly the aeroplane properly then it performs fairly well in coarse pitch as of the last steam patch. Most of the problem is that the sort of people who fly with CEM switched off don't know how to fly the aeroplane properly in the first place and have unrealistic expectations. The aeroplane weighs getting on for 3 tonnes and as modelled only has about 880 bhp at 3000 rpm & +6¼ psi boost at sea level. It's not an F-22.
The engine is optimised for combat at 16000' or so, and gradually improves in performance as you climb.
However, the aeroplane has plenty of thrust available if you fly it properly. I think what happens is that lots of people just haul back on the stick and expect the aeroplane to climb vertically or something. It won't.
You should only fly slower than about 170 mph IAS in a Spitfire I for takeoff and landing. If you pull too hard and get too slow then the aeroplane won't perform.
If you fly at 170-180 mph IAS then it climbs perfectly nicely in coarse pitch at +6¼ psi boost.
Once you get to operational altitude (16000-18000') then you'll see that the engine rpm is considerably higher than at sea level due to the reduced air density, and the aeroplane will perform as advertised. You should be able to attain about 260 mph IAS in level flight in this altitude bracket, which is roughly 350 mph TAS.
If you turn CEM on then you can climb in fine pitch which will improve performance at low level and lower airspeed; but if you exceed 3000 rpm for any length of time, or use excessive boost at low rpm etc. then you risk breaking your engine.
The Spitfire Ia should have a constant speed propeller, but it doesn't; this is a documented bug.
However, the Spitfire II does have a constant speed propeller, and so with CEM off you'll get 3000 rpm basically all the time, which results in considerably better performance, especially at low altitude.
In fact, it may be slightly too good. But that's another debate.
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