Takes 15 seconds before scroll bars and links become usable

When I switch to a new page--either from the local disk or from the internet--the image appears quickly but then it typically takes 15 seconds before the links or scroll bars become active. For example, after the 'Start new thread' appeared, it then took about 15 seconds before it was functional, i.e. before clicking it would do anything.

If this a fairly common problem? What's the usual cause? How do you suggest I try to find the cause?

(Note that since this problem happens regardless of whether the page was retrieved locally or from the internet, I don't believe this is a network problem.)

Windows 10 Pro Version 20H2

When did the problem start? If you know this then think about what you did at that time that may have caused it. Maybe a faulty Windows update. Installed a new program? Ran a registry cleaner? Accidentally installed a PUP or caught a virus from a dodgy website?