When I run a BIRT report for the first time, it takes a while to get to the progress bar, then a while to finally render the report. Every time I view the same report after that initial report, it goes much much faster.
It is like something is getting compiled on that first run.
we also observed the same behaviour. we are running in a server environment (multiple reports can run parallelly). The first report though will take some time additionally... may be bringing up the engine (if you are re-using the engine)