Profile your app using Adobe Scout. It's a free profiler for Flash that you can install from Adobe Creative Cloud. Basically the UI freezes if the system can't maintain a decent frame rate. You can read more about it in the Getting started with Adobe Scout article from Adobe. Here's a quote:
The most important thing to notice is the red line. This represents the budget time that you have to keep below on each frame, if you’re to meet the framerate you want to achieve. For example, on a SWF running at 30 frames per second (fps), you have a budget of 33ms per frame (1000ms / 30 frames). In other words, if you spend more than 33ms per frame, your application will start stuttering and skipping frames, which you really don’t want to happen!