Thanks for the reply Bob. I just put a new JD belt on it. Only the blade speed. I’ll run those checks and post.
Is just the blade speed slowing down or engine speed also? If engine speed also, are you running wide open throttle?
If only blades are slowing, with engine OFF, gently pull on drive belt to get a "feel" for how tight the belt is when raised. Lower deck and check belt tension again. Then, remove belt and try to wiggle/move blade pulleys and idler pulley...may be an issue with each or all ! Could also be a worn belt. JD belt or aftermarket? Check things out and post results. Bob