The problem with many myths is that they’re vague. Are certified refurbished devices slower? It depends — slower than what? A certified refurbished MacBook Pro that’s a few years old may be sower than the latest model. But if you buy a refurbished version of the latest model, there will be no difference in speed from a brand new one.
Part of the reason for that is that Apple automatically replaces any battery in a certified refurbished product with a brand new one before it’s put up for sale. Any part that has a negative impact on performance or speed will also be identified during the inspection and repaired or replaced.