First, congrats on getting a performance assignment, that can be the most fun you've ever had. Plus, if you get good at it, all of your coworkers will think very highly of you.
What's your background? Do you have any measurement tools, like STROBE, or APA? Do you have the ability to find people in your shop that DO have these tools?
I will probably cry a little bit if you come back and say something like "No tools, just got picked at random to fix something I don't understand." That's so frustrating to be in that position.
The first thing you need to do is try to figure out WHY the program is slow, and WHAT would be considered an improvement. Does it run for seven hours? OR is it a fifteen minute job that everyone thinks should run faster?
Take a look at the output from the job and take note of the number of EXCPs for each DD. If one of them is in the millions, then it would be a good place to start looking.
The next step is to open up the code and get a feel for what the program is doing. That way, you can better decide which tuning/rewrite options would be the least amount of work.