Strange behavior with the virtual disk speed

Hi all,
Since a while I've been encountering strange behavior regarding the speed of the virtual disk.

I created a storage pool with LVM-Thin. This is on an SSD.

If I create a new VM and install (for example) Debian on it, nothing out of the ordinairy happens.
I just have disk performances of up to 500 Mb/s and I also get these speeds regularly.

But when I migrate a disk from a ZFS pool, or I change a setting of the disk of the Debian VM just mentioned (for example, I turn on SSD...

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