A few things to look at. First PSoS actually openes the file on the server, so it executes the OnFileOpen script. If this script has something that requires a user response, it will stop the script. I standard Show Custom Dialog won't be a problem because it will be disabled on the server, but if you have something else using a field and a pause statement, it will just sit there.
Next is context. PSoS really doesn't know the users context, so your script has to know how to navigate to the correct layout and then find the correct record set.
Last thought may be the number of unstored calc fields you are trying to Sum. Another method might be to use ExecuteSQL in a PSoS script and have it sum up the data instead of trying to sum unstored calc fields. If you were at Devcon there was a session that demonstrated this and I think a demo file you can look at.
Thanks for the reply !
It was due to SUM function on an unstored calculation which is dependent on other unstored calculations .I have used ExecuteSQL alongwith PSOS and then the process somewhat speeds up .
The really worry was that some of the unstored calculations were returning "?" due to poor programmatic skills (by someone else) .
I have sorted out.