Hi kylawl!
This is probably happening because we throttle viewport rendering whenever a user interacts with UI in the editor. It’s a way of shifting priorities to the part of your experience that needs to be most responsive. It seems that we give UI priority in the scenario you describe, so holding down a button effectively freezes the game.
Ideally we would roll a fix for this into a bigger Slate/UMG initiative that’s currently underway. Hopefully this isn’t a blocker for you.