GUI Buttons -- how to have "return" key activate?

I've got a GUI dialog box with "return" set as the "accelerator" for the OK button, and "Esc" set as the accelerator for Cancel. I didn't used to think it worked at all, until today I realized it does work, it's just that you have to hold down the Return key for a full second before it does anything. Tapping it quickly or repeatedly has no effect.

This is not the desired behavior. Am I stuck with this, or is there a setting I can change somewhere, to make the Return key activate it instantly, regardless of how quickly it was pressed and released?

This is on a Mac... not sure if Windows behavior is different.