I find that a steady slow heat up gives best results for both slumping and draping. Many do a two stage heat up with a slower initial rise to a temparature above the annealing point and then faster afterwards.
A slow steady increase in temperature allows all the glass to be nearly the same temperature both across the piece and from top to bottom.
Speed in draping, especially where there are different colours and thickness, promotes "wings" as any unevenness in the heat across the piece will translate into variable drape speeds. Allow all the glass to get to the same heat, by going slowly.
I proceed at the same speed from start to the forming temperature - no speed up at all. Yes, the glass will accept it a rapid heating to the target temperature, but it does not promote even slumps or drapes.