Transfer speed only 50 MB instead 110 MB earlier? Why?

Hello,

I have OMV 3.0.89 installed in a virtual machine on an ESXi. The OMV VM does have 4 CPUs and 4GB RAM.
Some months ago, when I installed OMV I was very happy with transfer speeds. Copy from NAS (Samba share) to my Windows PC was always around 105 MB/s.
But since some weeks I have a problem. When I want to copy a big file (i.e. 10 GB) from my NAS to my Windows PC it starts a few seconds with transfer speed of around 105 MB/s but then the speed always goes down to 40 - 50 MB/s. But why?
The network and the PC can't be the problem, cause I can transfer to and from other PCs in my network with full 112 MB/s. The ESXi server is also nearly idle and it can't be a problem of the ESXi cause I can also copy with full 112 MB/s to Windows VMs on the ESXi.

So where could be the problem? It must be anywhere in the OMV or in it's configure. I do have a very simple config and only SMB.
How could I find out what is slowing down my OMV?
Please help me, I don't want to buy a dedicated Synology NAS again

Nobody can help me?
Maybe there are any SMB parameters necessary to get 112 MB/s again? But it's strange, cause I did not change anything.