you made a strawman with "fast grass King" and a "slow grass King" and want to move discussion in that direction. brilliant strategy. the fact is - there were no grass specialists for federer at all to compete when he was winning wimbledons, while there were bunch of them for sampras in his time.
not to mention you never cared to explain logic how the era with grass specialists is weaker than era with no grass specialists?!
and because I'm in a good mood, free hint for you: grass is grass, physics of the grass game didn't change at all, the ball still stays low on slices, ball still bounces irregular from time to time, it is still much harder to move than on hard courts and clay. for the real reason for play style change (no specialists) you need to take a look at strings and rackets.
I'm not the one who introduced the concept of fast grass as somehow being a mitigating factor enabling a player with less merits to be regarded as greater than his superior. For me, grass is grass. If you want to find the greatest grass player of all time, look for who has won most on it. You'll then find you have a choice between Laver and Federer. Laver won more, but also had more opportunities. How you weigh that is up to you. Sampras, however, won less than Federer with exactly the same opportunities. As it is patently impossible to prove he faced stronger opponents, no circumstantial evidence to suggest that he did, and theories can be made in either direction, sticking to the merits seems the obvious solution.