AO 2017 was fast for sure but the rest is nonsense. It will depend a lot on the draws as well as conditions. Plus fitness.
Also, if clay is slow and it is, then Federer should have not made so many finals there. Clearly he is capable though the soft draws helped him a lot too, but the point is, he's perfectly fine playing on slow surfaces. It doesn't go against him. In fact, I'd say these days, now that he's older, perhaps he needs a bit more time to set up his shots properly. So if the court is, say, medium slow, it's helps him.