You're not looking for errors.
You're looking for what is causing the script to take a long time to process or gets stuck, causing the server to "concentrate" too hard on your script.
In other words, a script is like a recipe for cooking. The longer the script is, the more time it may take to get done. Even then, certain steps will take longer than other steps: searching a database or resizing an image will obviously take more time than formatting HTML output.
If the scripts access a database you may need to add indexes to the tables.
Analyze the HTTP access log file for exactly what URLs are being hit. That way you know what scripts are being used and can see what they are doing so you can figure out which ones are taking too long.
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