I've upgrade to Parallels 20.0.1 and after that MacOS Sequoia 15.0. My Windows 11 VM's Parallels tools are upgraded, now the network throughput of low (about 1/4 of max) and sometimes 0Kb. When I start another download/upload, the throughput runs again, until a few seconds as it goes back 0 again. This happens on internet downloads, as well as local SMB/AFP shares within the VM. This only happens in Shared Network setting for the Network adapter, when I set it to Wi-Fi all goes well. Multiple VM's with the same issue.
Well, installing update 20.1.0 seems to have fixed the issue, although it does not mention it specifically in de release notes.