Could it be that the provider is silently inserting RSTs into live flows?
He's talking about traffic inside a VPN also there doesn't seem to be a problem with the transfer stopping (which RSTs would cause) just slowing down.
Before walking down that road, however, are you sure you're not dealing with an MTU/TCP-MSS/DF bit issue? I'd suggest that this is a reasonable angle - I've seen various VPNs go slowly because of these kinds of issues.
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