old belt = lack of power?

Can anyone tell me if an old belt with 141k on it would cause any loss of power from the engine? I know the belt needs to be replaced from a mileage standpoint. Should I look at the tensioner as well? If the tensioner needs to be replaced, would the "looseness" in the belt cause a lack of power...even if it is a little? Thanks!

the serpentine belt? negative. doesn't matter if it's baling twine, if it's spinning all the accessories like it should be (nothing is slipping, i.e. alternator is spinning and charging), you are good to go. change the belt tho before it strands you somewhere.