A site reported experiencing very baffleing symptoms with MYOB Premier opening and accessing it's datafiles. Some operations on the file would take between 40 and 45 seconds.
It turned out that the printer monitor program running on the windows clients was causing the problems. From the logs, we saw activity coming through with pauses of about 1 second.
Stopping the monitor software resulted in the networks access at normal (quick) speed. Restarting the program caused the speed to slow down again. The printer was a cannon lbp810 and the relevant task was something like CAPON (not sure on spelling). The monitor software displayed a printing now dialog on the client during printing.
We discovered this by starting with a clean install of windows and trying the app at every step of the installation of other software process (had to do this many times).
Moral of the story, check everything (other software included)!