Softsynths (just like real ones) come in all flavors of price and capabilities. An inexpensive one can no more be expected to respond as well, or sound as good, as a cheap hardware synth. Add to that the possibly limited horsepower from a lesser laptop, that may be struggling to play back styles developed for modern 128 voice arrangers, and a generic driver for a cheap internal soundcard, and you have a recipe for high latency and poor timing.

Just like most things, you get what you pay for. Latencies can get as low as 1.5ms with some pro audio interfaces, and modern Core Duo based laptops can breeze through 128 voice styles. But expecting hardware responsiveness when using built in audio on perhaps older laptops is not reasonable, IMO.