Multitcore/ Multithreading does not work

[Rick2023]

I use Version 2.8.0 (64Bit) on a Windows 10 (64Bit) System. The System has a i7 with 6/12 cores. When importing a video, the the performance monitor shows that only one of the 12 is busy wiht the task. the other 11 cores are idling. The imported video is a MPEG-4 (H264) file. The file sizes are often above 15GB and the import time is above 20 min by ca. 850 fps. The performance monitor says the import has 8 Threads and the CPU workload is ca. 8%.
With all cores involved, the import should be much faster, as the system has 32GB RAM and runs on a NVME SSD.