will rand() cause performance issue

You can use timing to check the load that rand() adds. Try one test run of a batch of requests with rand and then one without.

 

 

http://devcentral.f5.com/wiki/iRules.timing.ashx

 

and:

 

http://devcentral.f5.com/Tutorials/TechTips/tabid/63/articleType/ArticleView/articleId/123/iRules-Optimization-101--05--Evaluating-iRule-Performance.aspx

 

 

Aaron