Thanks for checking the project. I also couldn't fault it on my 24" (iMac) but that has a more normal, lot lower, resolution of 1920x1200pixels (less than a sixth of the pixels of the 27").
I had made sure that the version I was using of MacOS 10.11.6 was fully updated. I cannot find how to further update the graphics driver. One forum stated "On Mac OS X the graphics driver is part of the operating system. You can only upgrade by upgrading to a newer version of the operating system."
Following up on that lead, on another partition of the SSD, I have installed the latest MacOS 10.15 (Catalina), and installed VideoPad. VideoPad does not falter even when I make the Sequence Preview very large. However, even when I had not undocked the Sequence Preview and so it is kept at a more reasonable size, I feel that the preview is not quite as smooth as when it is on 10.11.6.
In general, I feel that computers will often run the best/fastest when they have the version of the operating system that was installed on the computer when it was manufactured (updated to the last sub-version), rather than the current and often larger operating system.
I think I will continue to use VideoPad under MacOS 10.11.6 but make sure I don't exceed the window size that it is happy with.
Many thanks for your response. Greatly appreciated.