Forum Moderators: phranque
The answer to your question is of course it will slow down your webserver. How much depends on your configuration. Whether is slows it down enough to be a problem is your own judgement.
With 5-8 million pageviews a month you might want to consider putting the adserver on a different machine. You may also want to consider a different machine for an image server (both for ads and other website images).
You may also want to take a look at OASIS (available at sourceforge). My impression is that OASIS is more difficult to install but has better serving performance. There are also loads of commercial software packages (many available form hotscripts), or you might consider a service provider like Adserver from FastClick, or you might consider getting a specialty box like AdJuggler.