The behaviour is pretty individual to each car, it depends primaraly on the overall condition of the hydraulics. the height control valves can become sticky, this will slow down any reaction, this can usually be rectified by the application of some WD40 to them, several applications over several days is usually needed.
If you don't know where they are, look along the anti roll bars and you will see a clamp around them, follow the linkage and on the other end you will find the height control valve, lubricate the ball joints on the linkage, especially where it is connected to the valve, once they are nice and loose put some ordinary oil or grease on them.
I have three XMs and they all behave differently, as long as the suspension rises and stays in the correct position I wouldn't worry to much but the valve linkage would definitly benifit from some lubrication or they could seize up completely.