My Dell optiplex 380 has almost exact specs with XP except I added a Radeon video card. What troubles me is when your main screen loads you wait a second for Sset field to fill, than another second for Sact field to fill in demo mode. When my main (run) screen loads these fields are already filled. In plugin the DataReceivedHandlerEvent, read, write timeouts is set to 500ms, this should be instant 10ms-50ms. I can increase these for you but you would see a noticeable 1-2 second delay when turning spindle on, off and changing frequency. The on/off is a fixed string, the frequency does have a very small calculation before it sends and seems to be where your problem is.This may not even be your problem but another step in process of elimination.
Since our specs are almost identical I would try first to see what's slowing comp down before slowing plugin down to accommodate. I had Compac's in the past and remember they normally have many things running in background. Another difference between our setups is I use a UC100 but many successively use the UC300ETH5. Once again this is all theory and I could be wrong.
Dan