Welcome to Stack Exchange :)
Poor acceleration is sometimes caused by dirty spark plugs or clogged air filters. Keep in mind that the spark plugs are probably more difficult to reach on a 99 Subaru than most other cars since the Subaru has a boxer engine. But, if you feel up to it, you could try replacing the spark plugs ( and/or air filter if necessary ) and see if that helps the issue.
There are very few clues to go off of here though, poor acceleration could be caused by many different things. With only one symptom it's hard to diagnose the problem with any accuracy, and as costly as it can be, the solution here might be to take it to Subaru mechanic.
Hesitation or low power at slow acceleration