Screen Refresh while Editing Theme

When I’m defining widgets and I type in one of the many types of text entry box, the theme preview adjusts to what I am typing.

Whilst that is fine in theory, in practice it is a real hindrance because after you tye a few characters you have to stop and wait for the preview to catch up. If you don’t wait any further typed characters are lost.

Is there a way to toggle immediate preview off till text box entry has been completed please?