Theory regarding the slowing of normals across the game - I think the creators intended to do 2 things with this change:
1) Slower startups, particularly on mediums, were intended so that players can whiff punish on reaction more easily.
2) Added recovery frames so that players had a better buffer to properly react to a 'counter hit' notifcation.
For starters, its not like SF5 is the fastest SF game to begin with, that honor going to 3rd strike. However, this goes more in line with some of the proposed design direction they were talking about even before the game launched. I do feel that these slower normals are what is causing players to believe the game has slowed down (technically speaking, it has).
This also serves to 'quell' the feedback that the footsie game in SFV was a shadow of what it is in earlier SF games, thanks in part because of the larger input delay SFV has. So, rather than nerfing the input delay, which probably exists as both a gameplay decision and for online matches and how the game exchanges data between users, they just decided to slow down normals on either startup or recovery.
I don't think Capcom is gonna necessarily win themselves any fans with these changes to the standards of mediums & heavies in the short-term, because players got used to those buttons being a certain universal speed for nearly a year, and now that is all upended. In the long run though, I do think this will make the footsie/reactionary parts of SF gameplay more accessible than it either was previously or has been historically for an SF title, without making the game cater exclusively to how speedy your reactions are. Hopefully this'll be the last time they introduce a major change to the baseline speed of medium & heavy normals.