Linux and large Files in Davinci -> poor i/o

I was investigating poor playback in Resolve Studio that plagued me for a while now (but not since forever) and stumbled across the following:

The longer the file is i try to play, the worse gets the performance (with the exception of one prores422HQ file). The behavior is unique to Resolve and BMD RAW-Player on this specific system. Other Software plays everything smooth as expected. Also does Resolve on two other Linux Workstations (Ryzen5600/RTX3060/64GBDDR4/PCIE4.0NVME + HPZ440/QuadroK6000/64GBDDR4).

After fiddling around with all kinds of cache settings, GPU drivers, Kernels & Resolve versions I did following test: playing a variety of files from different storage devices directly in the media window in Resolve after importing them to the project. I hit play for 10 s -> take the value from iotop -> skip halfway through the file -> play 10s -> take value -> skip to end -> play 10s -> take value. When iotop did not settle on a value i noted min~max. During this test CPU never hit 10% neither did the GPU. Here are the results:

USB3.0 HDD exfat-fuse (118,0 MB/s) [iotop: start | mid | end]

prores422HQ1080p25 (45 min)-> [30 M/s | 29 M/s | 31 M/s] -> smooth
braw8:1 2160p25 (45 min) -> [42 M/s | 5 M/s | 1~4 M/s] -> starts smooth - gets worse
braw8:1 2160p25 (10min) -> [0 M/s | 0 M/s | 0 M/s] -> smooth
DNxHD 185X 1080p25 (1h) -> [30 M/s | 4~10 M/s | 1~8 M/s] -> starts smooth - gets worse

USB3.0 SSD exfat-fuse (438,4 MB/s) [iotop: start | mid | end]

prores422HQ1080p25 (45 min) [20 M/s | start 6 -> stab. 20 M/s | 3 ~ 12 M/s] -> starts smooth - gets worse
braw8:1 2160p25 (45 min) [40 M/s | 13 ~ 33 M/s | 2 ~ 10 M/s] -> starts smooth - gets worse
braw8:1 2160p25 (10min) -> [40 M/s | 40 M/s | 40 M/s] -> smooth
DNxHD 185X 1080p25 (1h) [22M/s | 3 ~ 12 M/s | 1 ~ 8 M/s] -> starts smooth - gets worse

SATA6 SSD ext4 (561,1 MB/s) [iotop: start | mid | end]

prores422HQ1080p25 (45 min)-> [20 M/s | 20 M/s | 20 M/s] -> smooth
braw8:1 2160p25 (45 min) -> [55 M/s | 40 M/s | 40 M/s] -> smooth
braw8:1 2160p25 (10min) -> [40 M/s | 40 M/s | 40 M/s] -> smooth
DNxHD 185X 1080p25 (1h) -> [38 M/s | 38 M/s | 38 M/s] -> smooth

Can anybody explain whats going on here? How can i get back to smooth playback? What am i missing? Can anybody confirm this behaviour? Thanks for your Feedback.