I've seen/fixed similar issue in someone else's Windows 10 notebook few days ago. It seems to have been caused by wifi having the same ssid for 2.4 GHz and 5 GHz and not-that-great signal so OS was for some reason "constantly" switching between those two. After forcing it to use just 5 GHz it went from 1 Mbps to 100 Mbps. Reading this sooner would have prevented my headache.
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I've seen/fixed similar issue in someone else's Windows 10 notebook few days ago. It seems to have been caused by wifi having the same ssid for 2.4 GHz and 5 GHz and not-that-great signal so OS was for some reason "constantly" switching between those two. After forcing it to use just 5 GHz it went from 1 Mbps to 100 Mbps. Reading this sooner would have prevented my headache.