Serial traffic hoses virtual machine interactive performance?

[Lars Kellogg-Stedman]

I have a pair of virtual machines connected via a serial link. Machine
`node0.virt` is configured like this:

    <serial type="unix">
      <source mode="bind" path="/tmp/serial0"/>
      <target type="isa-serial" port="1">
        <model name="isa-serial"/>
      </target>
      <alias name="serial1"/>
    </serial>

And machine `node1.virt` is configured like this:

    <serial type="unix">
      <source mode="connect" path="/tmp/serial0"/>
      <target type="isa-serial" port="1">
        <model name="isa-serial"/>
      </target>
      <alias name="serial1"/>
    </serial>

If I have any sort of file transfer running over the serial link
(e.g., a zmodem file transfer over raw serial, or an `scp` over a SLIP
or PPP connection), the interactive performance of the *receiving*
machine (accessed via the console or over a network connection) tanks
-- it becomes almost unusable until the file transfer completes (or is
cancelled).

What's going on there, and is there a way to improve the performance?

(Same behavior when connected using a pty device instead of a unix
socket.)

-- 
Lars Kellogg-Stedman <lars at redhat.com> | larsks @ {irc,twitter,github}
http://blog.oddbit.com/                | N1LKS