Note: Use <code></code> tags around your code to keep it legible.

One single event isn't enough to react to updates and repaint the whole window among responding to all the minor events Windows may throw at your app. Put the DoOneEvent inside the loop you didn't show, rather than the if. That way, you'll interleave small bits of GUI action between record processing, and they'll both share the CPU.

Also, look up the parameters for DoOneEvent to find the best set of options for your situation. Not waiting if there are no pending events, for example.