DeckLink Mini Recorder 4K reduces Framerate

Hello community,
I have an issue with the DeckLink Mini Recorder 4K Grabber.

Based on a 1080p50 signal I tested two scenarios, it is always Ubuntu 18.04 and the SDK/Driver 11.7.
This grabber in a pretty new PC with a current Ryzen CPU, 16GB DDR4 RAM etc.
works just fine, I get 50 frames every second by the SDK.

I used the same grabber (I tested multiple DeckLink Mini Recorder 4K Grabber in multiple old PCs, it is not related to only one of them) in an older PC, i5 second generation four cores, 4GB RAM...
With this setting I get 25 frames at max, sometimes even less, while the CPU is not very busy.
The same happens if I use Mediaexpress instead of the SDK, the Video is recorded with 25 instead of 50 fps while the new PC is doing 50 just fine.

My question is, is this Grabber adapting to the system? And is there a way to suppress this?
BR Michael

How are you encoding it? The disk may not be fast enough.

Thank you for your comment.
The encoding is not important in this case, I even tried it without writing the file, so just capturing and don't do anything with the frame -> same result, 25 fps.
Usually I encode it with H264 on the GPU, the file size is pretty small and the SSD can handle this

My GPU is not displaying in Davinci. What's my issue. I have nvidia gforce