I noticed a strange quirk when doing things with click and drag in UMG while the game is windowed, and it only happens in a shipping build. It appears that, when either the mouse cursor is visible, OR the mouse is not captured to the viewport, a very low precision click and drag detection is used.
For moving the camera around a character in a menu this is easily avoided by setting Hide Cursor During Capture to true. For a slider, this is overridden with the cursor always visible causing the low precision mode to be used all the time. The result is a very laggy slider that is impossible to be precise with.
As for why this doesn’t happen in the editor: my theory is because the editor is running maximized in the background and overrides the windowed game.