Pack Slip Report runs slow usually only at the first time

 We have a customized Pack Slip report. The report can take 5 miniutes to finish, usually only when the first time you run it. If you run the report again on the same computer, it only takes 3 to 4 seconds. So it is hard for me to adjust and test the report performance because the slowness only happened once.

Is there anyone that knows how to tune a report? The slowness happens usually only at the first time, does it mean later on the report is cached in the memory?

thanks for your help.