Thank you for the post and I apologize for the delay. I do not have an immediate answer, so I wanted to see if anyone else would chime in with a similar experience.
Some questions to assist us with narrow this down:
1. What is the version of FileMaker Server 13?
2. What is the operating system of the computer?
3. What are the hardware specifications for the computer?
4. If the script does not use the Perform Script on Server command, but run locally with FileMaker Pro, then does the client experience a slowdown?
5. Could you post a screenshot of the script for review?
“Is there a way to know how many scripts are running on the server ?”
In the Admin Console > Statistics > Click “Statistics… > Check “Remote Calls/sec” & “Remote Calls In Progress.”
How many “Remote Calls In Progress” does the Admin Console report at the peak of the slowness?