Why is my transmission sluggish?

I have a 1995 Kia Avella (Ford Aspire) with an automatic transmission. The transmission works fine unless I let the car sit un-driven for 3 days or more. When I go to drive the car after 3 or more days, this happens: I shift into gear (reverse or drive) and press the accelerator. The car moves very slowly for a few seconds but finally operates normally. This only happens when I initially start the car, and doesn't happen again unless I let the car sit for a few days. It's like the tranny has to build up pressure or something...? This has been happening since about 2008.