It may well be totally irrelevant, but, if the problem is confined to Windows users,
this page, under the heading 'Installation location', is interesting. By default in Windows 10, SD4 has the QT executable installed in a folder called "Site Designer" (C:\Program Files\CoffeeCup Software\Site Designer\).
I wonder whether there are Windows users who installed SD4 in a sub- folder with no space in its name, and who use SD4 for hours without trouble. If so, perhaps the problem is not a programming one at all, but rather a matter of where users install SD4.
Frank