What would cause slow acceleration on a 1999 ford escort?

Lots of things, most of them engine problems ( assuming you your parking brake is

not set on and your tires all have air.)

The cheapest to fix would be badly adjusted or worn carbuerator and/or spark plugs and

spark plug wires - this is what the typical engine tune-up would fix.

More serious engine probles such as bad piston rings or warped valves would cause

slow acceleration and would require an over-haul.