The "bat speed" issue with Riley was really a "swing length" issue.
There is a good explanation of it in Kiley's article about Riley's call up:
https://blogs.fangraphs.com/called-up-austin-riley/
While his stats have been standout every stop of his career, his deep hand load, which helps create the separation from his body that manufactures the power, was getting too deep and sometimes barring out his front arm, which was changing his path to the ball. He’s been working on consistently keeping that front arm flexed, which helps him maintain the athleticism and bat control in his swing to get around on hard stuff on his hands and have better plate coverage.
Looking at the slow video it is easy to see Riley's hands go back towards the catcher before he swings. That's "deep hand load", and is what causes his swing to be long if it gets too deep.
Compare that to other swings of MLB players, and you will notice some don't have the dramatic movement that lengthens their swings, and some do. It seems to me that's something that can be tinkered with, and it looks like Riley has already successfully tinkered with it.