I'm sorry. It could very well be that your hyper-v is running linux in full-virtualization mode (full is bad). If you ran the client OS in para-virtualization mode, then it would run near full speed. I am not sure if hyper-v supports that or what. Linux is happy to run in para-virtualization mode, so most VM systems can do that just fine.
Yes, that's the problem. Microsoft's para-virtualization support for Linux is minimal. It supposedly works for Red Hat and Debian (

) but I've had no joy with Ubuntu.
Perhaps I'll set up some Red Hat VMs. There's another project that needs a virtual network of Red Hat VMs.
Or perhaps Win XP VMs, if I can dig up enough full-install packages.
Thanks, all, for your help.