I would say this is more proof of stronger competition that brought out what happens with every ATG under such conditions: random losses. More off-days. Being made to look bad when you're actually playing well (W 2015 in particular).no, that's just weaker competition making it look that way
prime fed does not get hammered in straights at AO as Nadal did to him in AO 14 or lose to frickin seppi in 4 sets
prime fed does not lose to gulbis in 5 sets at RG or get thrashed by Stan in straights without breaking him even once
prime fed does not return as badly as he did in Wim 14 final or hit as weakly off the ground (thanks to racquet change, I know) or play at a high level only for 2 sets (Wim 15 final)
prime fed does not nearly lose to Monfils in 4 sets at USO or get beaten in straights by Cilic. would certainly play better and not choke as badly as he did in USO 15 final (yes, USO 09 final which was the worst at his prime - was significantly better)
AO 04-10 Fed (minus 08) was way better than Fed at AO 17/18.
played well at Wim 17. would rate it higher than WIm 15 though not by much., but still lesser than Wim 12 QF-F, which was still post prime IMO (at best prime-ish)
Fed 11-12 was better than 14-15 and 17-18.
I think you think Federer is only playing at his best when he wins every major he enters and loses only against GOATing Safin or Del Potro, or in RG finals to Nadal, which is imho a fairytale.