A car will run slower once a body is put on as per the weight. I have the same body, both red and silver, and have had no problems with them. Try to make sure that no parts of the body are binding against any of the tires.

When problems like this happen you need to start asking questions to figuring it out.

First, what could the body change to cause it to go slower?

Look for friction points. Do all the tires spin equally before and after putting on the body?

Do you have the right front clip on? Do you have the chasis set to the proper length?