ZFS on FreeBSD 11.3 slower than 10.4

[Miroslav Lachman]

I upgraded my old desktop computer few month ago from old 10.4 based 
PC-BSD to stock FreeBSD 11.3. It uses single 2TB HDD 7200rpm.
My problem is that upgraded version is really slow and some desktop 
applications are very lagging (playing multimedia is interrupted for a 
fraction of seconds) when there is heavy filesystem activity.

I am using zfsnap2 for taking snapshots periodically and when there is 
enough snapshots zfs destroy is called. In this time the user experience 
is terrible. Starting new application like browser or even something 
much smaller takes minutes. The old version based on FreeBSD 10.4 
behaves much better. I used the old version for years and never have 
problems with interrupted multimedia playback.

Are there some sysctls to tune to get better desktop interactivity in 
heavy filesystem operations like zfs destroy, pkg check or other "find" 
periodic scripts?

Kind regards
Miroslav Lachman