Unstable transfer speeds

[Humberd]

Recently I've upgraded qB from 3.xx to 4.0.4 and I have very unregular transfer speeds.
https://imgur.com/gallery/HG5wb
Here are ther screens. The disk is 100% load all the time, but R/W speads are 1 MB/s. in qB downloads are jumping from 2MB/s to 0 every other second. And I have 0 upload speed, but I set it uncapped.
What is the problem?

@edit When I quit qB the process just hangs for a long time having 1MB/s R/W

@edit2 After manually terminating a process and opening qB again it needs to check all files that were downloading again.
After first few checked torrents everything seems to be fine, but later on it begins to have the same behaviour as described above.

@edit3 https://i.imgur.com/vh1wBZ5.png This is a characteristic pattern. After restart (left) the speeds went up. But after 15 minutes upload went to 0, and downlload has a 1 minute spikes

[Switeck]

This is probably due to:

High ram usage and disabling the OS cache cripple my download speed
https://github.com/qbittorrent/qBittorrent/issues/8630

The message thread gives partial workarounds to the issue, and a much better fix is already implemented into the latest libtorrent...which may appear in the NEXT qBitTorrent release.

Other people with the same or similar problems:
https://github.com/qbittorrent/qBittorr ... -371620895
https://github.com/qbittorrent/qBittorr ... -370217979
https://github.com/qbittorrent/qBittorr ... -346226997
https://github.com/qbittorrent/qBittorrent/issues/7888
https://github.com/qbittorrent/qBittorrent/issues/8357

There are also other problems which can reduce speeds considerably...
https://github.com/qbittorrent/qBittorr ... -372126618
https://github.com/qbittorrent/qBittorr ... -317431680
https://github.com/qbittorrent/qBittorr ... -348767823