Just wondering, when you're referring to WAN , is this WAN over a VPN connection? or straight up internet connection? IMHO, doing the latter, is a pretty bad idea, I've done quite a bit of experimentation on this. The best I could run reliably is an C/S ODBC which is ok, but the Sage client itself has too many variables that could cause lags.
I'm sure you've seen this article before, but as you can see, even Sage explicitly states that WAN Connections aren't supported :
https://support.na.sage.com/selfservice/viewContent.do?externalId=10472&sliceId=1
With that said, if this setup is the only way to go, I would tend to concur with @ChrisMengerink , if the new server has moved hardware (or even VMs which are using a different underlying Network Adapter. I would play with MTU, TCP Offload, etc...