1 FPS ore less in the "Manual Cache" Overview

Hello,

i have a little problem with the overview in the “manual cache” window. When I have marked an area for pre-loading and the download is finished, I only have 1 FPS and less in the overview. As soon as I’m out of the overview I have 90+ FPS again. I only have the problem in the selection window with the world map. Have already deleted the area again and downloaded again. As soon as I delete the loaded map section, I have no FPS problems again.

The computer should have enough power for this and the HDD is not the oldest either.

My computer:
Intel (R) Core ™ i9-9900K CPU @ 3.60GHz
Z390 AORUS ULTRA
NVIDIA GeForce RTX 2080 Ti
16GB Ram (2x8GB)
WD Black WD4004FZWX - 4TB 7200rpm 128MB (hard drive where the cache is stored)
Cache size: 500GB
Size of the loaded map 11,24GB

The MFS 2020 is installed on an SSD. It would be nice if someone had an idea that could solve the problem.