The T-1000 wasted no more than two or three seconds impaling the trucker on its arm, and if it were in any great hurry, it logically should've been running towards the truck, rather than walking. Its behaviour suggests it wasn't in any great rush at that point.
As for what it gained by killing the trucker, perhaps it wanted to ensure that the trucker wouldn't have the chance to alert the authorities as to the theft of his truck. The T-1000 didn't have much to fear from the police in terms of its own safety, but coming under fire from the police while still pursuing its targets would likely have proved at least a minor hindrance. The scene is reminiscent of an earlier scene where The T-1000 either gut-punched or stabbed (likely the latter) a police officer before stealing his car, presumably for the same reason.
Edit: In addition to what I wrote above, I think Jenayah was correct in suggesting that the T-1000 had a rudimentary personality. Jenayah cited the T-1000 wagging its finger at Sarah Connor in the steel mill, which hints at a limited sense of humour. And after hurling the T-800 through a window during the fight in the mall, the T-1000 paused to look at a silver mannequin, frowning slightly, before continuing its pursuit of John.
Now I don't necessarily think it killed the truck driver out of irritation, since it dispatched that police officer in a similar fashion shortly after arriving in 1995, when it had no apparent reason to be annoyed. Seems more likely that it was just silencing a potential witness to its actions, in both cases. However, if the T-1000 did indeed possess a rudimentary personality of some sort, it follows that it wasn't wholly single-minded in its pursuit of its mission objectives. It had some degree of free will, and could make choices that didn't strictly benefit its mission.
I think this can reasonably be inferred from its actions within the movie itself, but the novelisation is more explicit:
"it was fully autonomous, and barely under the allegiance of its creator, SKYNET. SKYNET had hesitated before creating this latest weapon system. There were unpredictability factors related to the liquid poly-mimetic alloy's longevity and the ability to process commands without interpolating its own priorities over those of its creator. it was so volatile a construct that only in the last throes of defeat, only when it appeared that the Resistance would finally be able to mount an offensive against the inner command components of SKYNET, even threatening the Cheyenne Mountain complex itself, did SKYNET go ahead and create the T1000. Einstein once said that God didn't play dice with the universe. SKYNET had no choice..."
Sourced from: http://www.jamescamerononline.com/T2FAQ.htm