Recently I've installed Ubuntu 18.04 and I've noticed that sharing a samba folder from a Linux server over a openvpn tunnel is very very slow. Tried to avoid using DNS, but still have no results.
What I can say is using a Mac OSx sierra or a Windows OS laptop, the whole service is fast and speedy. That means is not the connection but Linux in someway and I've read lots of pages (dated 2012 maybe earlier) about people having same problem but no real solution.
VPN uses a 192.168.x.x IP and after a lots of tries (MSU, PINGs, ecc.) the only improve I could make (reading this form) was changing my LAN IP network from 192.168.x.x to 191.168.x.x and install cifs-utils: now my connection is a bit faster and I can open files from that folder without wait 5 minutes but still very slow comparing Mac and Windows.