So now I actually do not think the disk is the problem.
I emptied the disk and ran a couple of copy tests again, this time no speed decrease at all.
@itimpi
I also ran the benchmark as asked, I don't see any issue here either (although it looks like it does only read tests?)
root@Kratos:/mnt/user/testabc# rsync -av --progress /mnt/user/plex/TV\ Shows/House/Season\ 8/* ./
sending incremental file list
S08E01 - Twenty Vicodin Bluray-1080p.mkv
3,519,696,181 100% 143.10MB/s 0:00:23 (xfr#1, to-chk=21/22)
S08E02 - Transplant Bluray-1080p.mkv
3,519,986,886 100% 124.77MB/s 0:00:26 (xfr#2, to-chk=20/22)
S08E03 - Charity Case Bluray-1080p.mkv
3,519,238,379 100% 76.13MB/s 0:00:44 (xfr#3, to-chk=19/22)
S08E04 - Risky Business Bluray-1080p.mkv
2,617,081,856 74% 91.85MB/s 0:00:09
kratos-diagnostics-20200711-0831.zip