Uploading files from computer slows internet access for entire home network

​​​I'm a video editor and we have Xfinity's "Superfast" internet access with unlimited monthly upload capacity.​​​

​​​Problem: lately, whenever I attempt to upload a large (multi-GB) file from my Windows 11 PC, internet access for our entire home slows to a crawl, including WiFi and streaming via Apple TV (which is hard-wired to our Xfinity modem).​​​

​​​Service is immediately restored as soon as I stop or pause the upload.​​​

​​​Why?​​​

​​​I know upload speeds are throttled, which is fine, but why does uploading so dramatically affect download speeds for the rest of the network?​​​