Supposedly Borg had one of the lowest resting heart rates among athletes at 35 bpm. The guy retired way too early though to be considered the GOAT but he is certainly in my top 10
Well, he kinda went crazy. But, I’ve never liked the longevity is a prerequisite for GOAT stance. Barry Sanders is certainly in the discussion when it comes to RBs and he retired when he was 30. On the other hand, Jagr is going to play hockey seemingly forever and will almost certainly end up in second place in points behind the Great One. But, he’s not even close to the second best hockey player of all time.
Borg won 11 majors before the age of 26 then retired. Hard to compare eras - certainly one of the all time greats. Plus he had a number of failed business ventures - dinging him for those.
GOAT was Rod Laver in my opinion. Look up old videos of this guy. He was amazing. Great on all surfaces (they only played majors on grass and clay but it would not have mattered) and Laver won two Grand Slams, as an amateur in 1962 and as a professional in 1969.