Check the VPN concentraion point/path. Since multiple locations are experiencing the issue, the cause is likely rooted where there is a common connection — for example, the firewall/router, VPN concentrator/server.
Check duplex and speed settings between devices. Also check MTU across the WAN path. Are the links at the other sites already loaded by users doing streaming media or other file transfers? Do they have enough capacity to do what they need to do? What is the distance between the remote sites and the host site? Maybe they cannot get any faster throughput if the latency is already high.