Not only is df+LK slower, its hitbox and airborne frames have changed.
It is more likely to trade now than it was before, and it is now airborne in its active frames, meaning if you trade (i.e. get hit during its active frames), Chun gets air reset (she does the little bounceback in the air before landing on her feet). The air reset makes it almost impossible to trade and follow up with anything, unless your opponent is in the corner.
It is still of some use as an anti-air, but it has now become situational like all her other anti-airs.
Also, regarding characters falling out of Ultra 1:
A large part of this has to do with spacing. For example, there are certain characters where, when spaced JUST the right distance away from the corner, will still juggle properly with Ultra 1 in Vanilla and in Super. However, in AE, when a normally juggle-able character gets Ultra’d at that distance, the launcher will connect but not all the hits of the follow-up, the rising whirlwind kicks, will connect. In some cases the opponent will drop out before the final hit of the Ultra.
Also, far HK feels slower to me. I might be wrong, but it sure feels that way.