How do I handle my difficult co-worker?

Okay I know I have run this question into the ground because people who have read my past questions know I already have a new job waiting for me in a week with more money. BUT my friend I work with at my current job is still bugging the hell out of me. We work at Potbelly's, and yesterday she told me ';You need to do more multi-tasking.'; So today I made sure I was ahead of my step and did everything in my power to do my best. So at the end of the day she told me ';Well you did good but you still could have done more- Potbelly managers want us to really go all out.'; My thing is- am I just bad at fast food retail? My new job is as a tour guide- since I am a film/theater major, I am a very creative person, so retail jobs bore me, and maybe the other co-workers dont like this about me so they get an attitude. My friend- Christina- is also a ';saved'; Christian, who is over-the-top at Potbelly and a perfectionist. No matter how good I try to do she always nags I could do better