I'm experiencing the same issue as well. When I launch vacfs on the same machine as Venti, reading is very slow. When I launch vacfs on another Plan 9 or Unix machine, reading is fast.
I've just made some measurements when reading a file: Vacfs running on the same machine as Venti: 151 KB/s Vacfs running on another machine: 5131 KB/s -- David du Colombier