waiting during transaction

Hi, I have made a table with statistics of my visitors. Now I have very
complex views(query's) based on it. The datarequest takes too long to
process(longer than 5 seconds...) for a website. I want to show the user a
waiting screen(something like "retrieving data...") in stead of just a blank
screen. I probably have to work with threads and some communication between
server and client. However, I know very little about the topic. Any pointers
or a good website that discusses the topic would be welcome.

Kind regards,
Nico.