I actually think this rule will save as much time as the pitch clocks. Did you catch that stepping off the rubber also counts as a pickoff move? Think of the pitcher-batter confrontations where they stare at each other for 20 seconds then step off. I bet there are 10+ step offs each game for each team.
Now there will be little need for the batter to call time because the pitcher is taking too long. And the pitcher simply has to throw the ball rather than step off the rubber. It will take a few months but I think pitchers will just get a rhythm of getting the ball and throwing it. If there are more stolen bases then so be it. There will be no more "that is not his good move" pickoff attempts. No more stare downs. A 10-pitch at bat could actually get done in less than 5 minutes with men on base (assuming foul balls which will always take more time).
Getting the MLB package this year and starting games an hour late has really given me insight into this rule. There is an easy 10-second skip on the app so you can see when a pitcher is in rhythm. Wheeler and Suarez are a joy to watch early in the game as they are very consistent with 10-12 seconds between pitches. When runners are on the rule is usually 2 clicks on playback (20 seconds). Some pitchers adhere to that. Some are not close to that (I am looking at you Kyle Gibson). Just about every reliever is around 20 seconds with nobody on base and 30 with runners. Then multiply times 2 or 3 with pickoffs and it is just terrible. One of the things I love about hockey on TV is that the last 10 minutes of the game move almost as quickly as the first 10 minutes (teams get only 1 timeout each and generally use that in the last minute). Baseball needs this.