Hi
Per your description, it looks like caused by the bandwidth.
What about the users in your datacenter site? Does the same problem happen to them? If not so, in my opinion, although your servers in datacenter with plenty of bandwidth, the two clients who built the connection were all in the branch site. So what about the VPN bandwidth between the branches site and the datacenter? Please check out whether It is stable or not. Does any packets is lost?
Regards!