Most of the info you want is here:
http://forum.1cpublishing.eu/showthread.php?t=21147
Basically what's happening is the CEM switches to coarse throttle after take off. You need to fly as level as possible when this happens to pick up speed/energy and then it's very important to maintain that energy throughout your flight - hard turns/climbs in the Spit will bleed it off and slow you down. Turn slow and smooth, climb slow and smooth and you will get used to it. As others have said in that thread, do try the CEM, at least for single prop planes (the twin engines are a bitch lol, but the singles are pretty easy) - I did, after encountering this 'bug' and have never looked back.
There is speculation that only the Spit 2a has the correct flight model, and that the Spit in the campaign is slow. The game FM's are being worked on every patch so I'm sure if this is the case, it'll get fixed soon.