The speed is not affected by the weather like DSL may be -- it's fiber, either you have it or you don't. However, in blizzard conditions, the fiber itself may well be running on phone poles, etc. for part of it's journey to your home and as such those connections could be subject to high winds, etc. and cause intermittent issues.
For speed, it's more likely that everyone is stuck at home due to the weather ... and well, with nothing else to do ... probably surfing the internet just like you. Networks are built for typical traffic loads, not the extremes (think shopping malls -- normally uncrowded but during Christmas season, no matter where and when you go, you'll hit the crowds).