I don't think it's a huge mistake. Despite unfreezing quicker, being separated from his constituent parts may have had some negative effect on the T-1000.
For instance, the T-1000 is made of mimetic poly-alloy, and likely (ok, so were entering speculation town, population: Me) relies on magnetic effects for some conduction effects. It may use gigantic magnetoresistance which relies on a soft magnet's spin being easily flipped at room temperature. Being a part for such a long time, without contact to the other pieces may have slowed it down somewhat.
Along with that as a poly-alloy it may not have had a completely metalic structure and relied on polymer aspects. If these aspects were semiconducting as some polymers are, that would have had a similar jarring effect. Not to mention it would have maybe damaged some of them on a molecular level
In summary, yes it'd likely that it was for that 'Hollywood Effect', but it's not impossible that smashing the T-1000 had it's own benefits for Arney.