Is this a turn by the autopilot with  the airspeed hold on ?

Are you at high altidude ?

Are you controling the rudder yourself, or is "auto-rudder" on ?

If you enter long turns at altitude, they MUST be coordinated and you MUST add power... because as you displace the lift vector into a turning vector, you'll need more power to hold altitude/airspeed.. especially in the thinner air at high altitude..

If you just roll into a 30* bank, try to hold altitude without more power and proper rudder input.. the airspeed will indeed fade away.

There are adjustments for parasitic and induced drag in the.cfg file.. but any changes there, that would help this situation, would have to be drastic and would mess up the rest of the performance envelope.

Does this happen to you in other jets ?  I'd be curious to see the cfg file for that plane...

For reference though.. at 3000 feet you SHOULD be going too slow for steep turns in a big jet. By that altitude, you should be established on final ...

If you can.. attach the cfg file to an email and send it to me and I'll see if there's anything unusual.