We have had several instances now where we've had an entire network segment (all devices, not just the Mac client) slow to a crawl. In every case this has been related to someone who upgraded their Mac with Sophos installed to Big Sur (against IT's advice). This is happening even after the GA release with supposed support for Big Sur. We've had to uninstall Sophos from these machines for now to work around this. Once we uninstall Sophos, the network returns to normal.