Ok I tried an old brick of a mechanical KVM and it now gives me the image just like with just the splitter. So the KVM definitely degrades the signal. It's one of these auto- no power switchable ones.

So I could source a better mechanical KVM. No issue.

But what about the splitter?

Thought I got a decent model, but with eBay it's hard to tell right 😀

If money is no object, get a VGA matrix (switch+splitter in one device).
The cheapest option would be to swap cables and test which have the least impact on signal quality.

As a general rule, switches and splitters with a higher bandwith should provide a better signal. Unfortunately manufacturers tend to list rather "optimistic" bandwith ratings for their devices. And finally, this is the first time I've heard of a mechanical switch performing better than an electronic one, usually it's the other way around.

Good idea to test the cables one by one, take a screenshot and compare 😀

Do you have a link to such a Matrix Switch?

I'm also thinking, I will mostly be doing HDMI capture on my SS7 machine. For the few VGA captures I can work on the desktop. The AVerMedia has surprisingly almost no delay / lag.

I might just have a very cheap KVM then. I tried with PSU, made no difference.

Images:

Just looking at the pictures, the switch looks cheap, but might actually not be complete crap. Those cables don't seem to be very well made though.

For matrix switches, just search "VGA matrix" on ebay. The good ones come in 19" rack format for several hundred dollars. The plasticky ones for 50 dollars are probably not much better than your current setup.

I think the Ferrit cores are just for low frequency damping, usually the running brightness stripes over an analog screen.
The contrast shadows are more a wrong impedance coupling leading to reflections.

A more suitable test is to choose a higher resolution and a black to white transition and see how the reflections get dampened. Then start changing components.
Be aware that already the output of the graphics card plays a role and has to match the impedance. So a card that puts out a blurry image with one setup may have a sharper output with a different setup that matches better.

I had cables without these ferries and they are absolutely shocking. They came with that KVM.

I tried all cables and directly to the capture card the image is flawless. Graphics card is a FX5200.