Throttle position sensor

What does the throttle position sensor have to do with this problem? "Won't drive very fast" is open to interpretation, and can mean a lot of different things. You could have a sticking brake, a plugged exhaust system, misfiring cylinders, etc. You need to provide some details or test results to analyze.

You aren't providing any information that I can use to help you. You haven't listed the diagnostic fault code number, engine size, what led someone to the throttle position sensor, etc. If your engine uses the very dangerous "throttle-by-wire" system that put Toyota in the news a few years ago, that is best left to your mechanic. That system is usually not repaired by replacing a simple, single part. Most of the time a complete accelerator pedal assembly or throttle body assembly is replaced.