I think Conner has lost some of his early career speed. He was never a speed guy, but he's pretty darn slow now.
I'll just Jerry Rice as an example. He hit multiple loss of speed plateaus, and by the end, he was too slow to be a viable NFL player. The Raiders Jerry Rice had already lost speed, but his game was never really a speed game.
The same comp can be made with Fitz. When Arians came in, he made Fitz a big slot because he wasn't fully a viable outside receiver. But at the end of his career Fitz hit another speed decline and was no longer viable at all.
Obviously this is all observational, but there are a number of examples of guys who lasted because speed wasn't their number one attribute.