Go at the speeds that give you the best quality of cut. If the machine sounds rough or quality suffers, slow it down. Haste makes waste. Unless you are doing production and really have to run fast, it just doesn't make sense. Even running 'fast' - especially in 3D - doesn't mean that it will in fact run all that much faster due to ramping, and the cost may be quality.
I talk a little about good speeds for 3D here in the Ramping Article.
-B