Hi furrycat, I was wondering when you'd drop in on this conversation! (for everyone else, Cmdr furrycat and I recently spent a very pleasant evening in a London beer garden discussing optimal planetary approaches in GREAT detail - I defer to his boundless knowledge in this matter). [yesnod]"Too fast" is based solely on descent rate. You can go faster at, say, -15 degrees than -45 degrees because the vertical component of your velocity vector is less.
At orbital cruise altitude, ie the blue circle, your descent rate may not exceed 200km/s, as indicated on the HUD. At the drop altitude it may not exceed 5km/s. At any given altitude between those ranges the maximum allowed rate scales according to what I suspect is an inverse square law but haven't tested enough. It is certainly not a linear interpolation.