That is a real risk - however, this time around people are already working on their own fast implementations (both software and hardware versions), so a lot of

That is a real risk - however, this time around people are already working on their own fast implementations (both software and hardware versions), so a lot of the dumb mistakes will already be avoided. There is also the risk of a lot of features being too slow to use at all in a fast encoder, but that's not a risk exclusive to AV1. HM (the reference H.265 encoder) was far too slow to be practical, and given the current speeds of H.265 encoders compared to H.264 at their best quality settings, it's not clear that they totally succeeded.