Slow connection over AFP

Hi,

I've just installed a nice new shiny Mac Mini running Yosemite and Server with Thunderbolt storage to replace a Windows SBS2008 box. However although most users are experiencing nice speeds , there are at least 3 that are struggling with slow speeds. So far I've discovered that one user is using Mountain Lion and an Ethernet cable,one is on Yosemite and Ethernet cable and the other is possibly on Yosemite via wifi.

However there is another user on Yosemite and Ethernet and no problems at all. One thing I have noticed is that on the server app, I've noticed some users are connected via AFP and some are SMB, however the 3 that are complaining of issues are all via AFP.

Can anyone please advise and suggest any changes

Without sounding stupid, what's the best way to get them connected via SMB. Connect to server and then smb://ip of server or is there some setting somewhere that I can force them to use SMB and connect via the pane in the finder? Or even disable a setting on the OSX server. I'm guessing I could just disable the AFP tick box in the shares in the server manager?

I knew they were getting rid of it but surprised that some are ok, so it's a bit hit and miss.