Preface: All of this is untested, lacking benchmarks, purely qualitative, and seriously failing at all aspects of scientific method or rigor.
The following commands should do something in theory. The powershell is for WSUS 4.0/4.1. You also need to open the GPO and check the Computer -> Admin Templates -> Network -> BITS template for the download speed restrictions. With the restrictions appropriately set (or removed if you’re impatient like me, just remember to set them back); bits can be set to foreground mode (same as foreground mode all the way back to WSUS 2.0).
Note that that command will only affect the in-memory instance of WSUS, restarting the service will wipe the setting. There’s a .SaveConfig() command you can run to persist the setting but I can’t attest to the effects.
Thanks to Tyrone Watt for the ps. BitsDownloadPriorityForeground isn’t listed in the IUpdateServerConfiguration interface, so god knows where the guy he got it from…. got it from… cause the link’s dead…and waybackmachine shows his guy used the old sqlcmd method (which doesn’t work for WSUS 4.x)… see why I do this?