Fixing increased CPU usage after installing .NET 4.8

Question

The situation
Hello, our Windows Forms application embeds a camera view through a third party library. This view also draws some graphics elements and refreshes with 30 fps. The CPU is quite old (Intel Core i7-6820EQ @ 2.80GHz) but everything ran fine with .NET 4.7.2. The single used CPU core of this 32 bit application is ~50 % here. The Windows version we are using is LTSC 1803 with the latest updates.

The problem
Now we only install .NET 4.8. CPU usage increases to ~70 % and the UI freezes. This worked with .NET 4.7.2.

The tests
What I've tried so far:

• Disable new accessibility features with "Switch.UseLegacyAccessibilityFeatures=true" for 0 to 4
• Looked at the release notes, looking for potential changes that would affect us (found nothing)
• Updated our application to use .NET 4.8 instead of 4.7.2
• The same behavior can be observed on several other (and newer) PCs with different hardware
• Extensive web-searching (found nothing)
• CPU usage performance checking didn't amount to anything. There are a lot of events from drawing the camera view but that's it.
• Updated to the latest Windows Updates. This reduced CPU load a bit but the UI still freezes.
• Performed a quick and dirty change to 64 bit. This reduced CPU load a tiny bit but the UI still freezes.
• RAM, disk, and GPU usage are all very low.

The question
Why is the UI frozen even though the CPU has ~35 % headroom?

Thanks for the help :-)