The Microsoft CIFS/SMB protocol is known for performing quite poor when running over links with more latency than a typical LAN:
http://www.networkworld.com/article/2344669/data-breach/why-cifs-and-mapi-are-poor-network-citizens.html :
CIFS runs slowly over a WAN because when a client requests a server to open, close, or read a file, CIFS breaks it into blocks and transmits the blocks sequentially rather than send the whole file,. For example, if a client requests a 1Mb file from a remote server, CIFS breaks the file into hundreds of data blocks and sends the blocks one at a time. The client acknowledges receipt of each block, and only after receiving the acknowledgement, does the server release the next block. This back and forth chattiness adds unwanted seconds, or even minutes to the simple task of opening the file.
And: https://blogs.technet.microsoft.com/neilcar/2004/10/26/smbcifs-performance-over-wan-links/
And: http://www.stevenjordan.net/2012/12/university-wisconsin-stout-wan-file.html
I'm not an expert on this topic, but it seems that newer client and server OS-ses should perform better; also this is the space where WAN optimizers play a role.
So first check if it is not the latency that kills you (which is likely); then try to find a solution from there...
Herman