Thank you olek for your comprehensive reply. I've been doing more testing and the root of my issues are discussed in length in this other support request:
Basically it's a discussion on a theme's border being too small and the difficulty with mouse selection when using Marco (No compositor.) All of my testing of 20.04 with Marco (Adaptive compositor) has been on a local area network with a 10gbe connection. (If anybody wondered, I can't tell the difference between 1ge and 10gbe when running VNC on my home network.)
When comparing 18.04 to 20.04, there are definite changes in how Marco (No compositor) works. For example, I'm using a borderless theme (Adapta) and it's still usable (but not great) in 18.04. In 20.04, it's not usable at all - I can't resize windows - there's nothing to grab onto.
For a very fast VNC experience, Marco (No compositor) is the only way to go. I tried all the included themes and there are only about two that work OK with this setting. Those are Traditional Green and Traditional OK. It seems that all the other themes have very narrow borders and are hard to select. To solve the problem for now, I modified my third party theme I've been using (Adapta) by adding a 5 pixel theme border. It's not pretty, but it gets the job done for now.
Going forward, to keep the Mate desktop relevant for remote access needs, we need Marco (No compositor) to work like Marco (Adaptive compositor) does in handling detection of window borders for resizing.
BTW, I also checked out XFCE and they have exactly the same issues.