File transfer performance is an end to end measure so in fairness to the ISP some effects are beyond their control. At the same time ISP's use it as an excuse not to improve back-haul and peering. Sounds like in your case the physical link is operating correctly but problem may be with how your ISP peers.
Not a lot you can do about your ISP's business practices.
Might try doing traceroute (tracert in Windows) to stable sites like BBR to see where problem lies.
Try increasing RWIN to partially compensate for latency. Check out the Tools page.
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