Showing a new screen is not instantaneous. There may be transitions (probably not in your case, or did you configure any screen transitions?), collections/factories need to load/unload (unless you have preloaded them) and there may already by an ongoing action (show/hide of a screen).
Monarch should take care of everything for you. The purpose of the queue is to handle a sequence of screens being shown or hidden. You don’t need to create your own queue.
I’d be very happy to look into this if you share the project or create a repro case.