How can I diagnose the cause of drastically slower time it takes to complete a "search files for a string" when using Zend Studio?

I frequently use Search function inside Zend Studio software (which is built on top of Eclipse). I use it a lot. Probably 10-100 times a day.

I have experienced a mysterious variety in search time, when I use Search to find a string in one of the files inside the Project. Often it takes just a second to complete the search request. But sometimes it takes 10-20 seconds, and that is the issue I seek to resolve/diagnose.

What I do:

• press Ctrl-H (opens up the search menu)
• type my search string
• press Search
• usually it takes about 1 second or less, but sometimes it takes 10-20 seconds [and that is the issue]

What I See

It happens sporadically, not all the time, but it happens frequently nonetheless.

What I've tried:

I have tried

• restricting Working Set to specific subfolders
• Restricted File name pattes to specific file extensions
• Defragged my drive
• Antivirus does not appear to be scaing
• Using a tool such as Process Monitor does not gather much info. i.e. last time I caught it I think svchost appeared to be making lots of file read requests, but that didn't tell me much.

Question

How can I diagnose a cause of the search slow-downs?