This may or may not be the problem, but my 627 pro would "drag" on DA trigger pull. When I took it to my smith, he told me that damn near every new S&W revolver he has seen as of date, the hand has been slightly out of timing with the CYL. It will spin the cyl, the cyl will lock to the next position, but the hand will keep trying to push against it making the trigger pull become more difficult.

The best way to see if this is your problem, open the cyl, manually move the thumb release to the "cyl lock" position, and make a DA trigger pull. If it is noticeably better, you have that issue. The hand is slightly out of timing. Once my smith worked on it, that problem went away completely. DA pull is now buttery smooth.