SSH is slow, when loggin in and when typing commands. Also, the OMV web interface is almost dead now, it is so slow. I've rebooted the server and there seems to be no change after the reboot
Maybe your flash drive sucks (which is pretty normal, the majority of these things does when used as an OS drive).
Can you please provide the output of these two commands (stopping the second one after 60 seconds with ctrl-c:
For iostat to work on x86 it most probably needs an apt-get install sysstat before. And while iostat is running, please click around in OMV's web UI.