This message was posted on behalf of Intel Corporation
We understand you're having bandwidth degradation after downloading large files using an Intel® Dual Band Wireless-AC 8260.
Please download the Intel® System Support Utility, then reproduce the issue. While your connection is degraded you can run the SSU tool and save the report.
Aside from this, we could also take a look at your WLAN report (Create WLAN Report in Windows 10 by tenforums.com).
NOTE: Any links provided for third party tools or sites are offered for your convenience and should not be viewed as an endorsement by Intel® of the content, products, or services offered there. We do not offer support for any third party tool mentioned here.
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However it should be simple to test if your router is the issue. For this you'll only need to download a large file while connected to a different network. If there is no issues then, we can recommend updating the firmware on your router, or changing it's Quality of Service (QoS) settings.