If it's slow to shift to a larger rear cog, then it might be as simple as adding some cable tension via the barrel adjuster. Parktool.com and bicycletutor.com are two of the numerous sites that explain these adjustments.
If shifting is hesitant in both directions, it's possible the interior of the cable housing ("liner") is worn or crudded up. The liner in the short section of housing nearest the rear derailleur is particularly prone to wear (due to its tighter loop) and contamination (due to its location).