I've just set up my server new with change from ubuntu server 18.04 to omv5 because of more easily managing it. It's running onto a HP Proliant Micro Gen8 with 12GB Ram, System-HDD is a SSD 500GB, Data onto 3x WD Red 4GB (Raid5).
I use the same shares as before to serve the data, only samba is active.
Since I serve the data with OMV5, the access to the shares is very slow, if I change from a folder into another, it needs few seconds until I can see anything.
If I use smb://... to access, all seems to work fine, only the access over mounted (fstab) drives is slow
Edit: I found it - clamav was the reason - after switch off clamav, the access to the folders and files was running as expected.