01 Mustang GT s slow

My initial guess is the clutch is slipping and bad. If you can take off and see normal use at a slow speed, light throttle and then floor the throttle and watch the tachometer go bonkers high and then speed catch up, it's most likely the clutch bad and/or poor adjustment. However, I'd suspect bad clutch. How many miles? Did the previous owner tell you anything? Did you test drive before $$$$? Of course, this is all provided the engine is in tip top working condition. btw, how many miles?

not enough info provided to give any meaningful feedback

verify first its not loosing fuel pressure