Confirming the issue. Guest performance suffers when the host is doing disk IO. This is most noticable in the form of audio stuttering. On a windows7 host (workstation 8.02), all of my guests have the same issue. Guest OS's range from windows 98 through windows 7, Redhat 7.3 through Fedora 16, and OSX 10.5 through OSX 10.7.3. The issue is worse when IO is occurring on a disk that is connected to my Jmicron controller than it is on my intel controller. Updating the host Jmicron driver does not resolve the issue.
I generally use guests for non-audio-related automated/scripted software testing, and as sandboxes. The issue doesn't cost me any productivity.
Bench system is an old i5 750 on an Asus P7P55 Deluxe.