I've found that with detailed carvings, the feed rate I set is pretty irrelevant. Since the toolpath consists of lots of tiny combined movements in Z plus X or Y, the controller will be slowed down a lot more than the nominal feedrate would indicate. If it comes to a straight stretch, then it will suddenly go much faster. If you provide your machine with a faster motion-control system, it will speed things up generally, but you can always expect a series of very short moves to take more time than long straight ones.