Yes, having a lot of different ads shown on your site will slow it down. The more images or anything that a user has to download will lead to slower loading times. For every banner ad or flash ad that you add to your site. Every visitor to your site will now have to load those ads. This results in a sluggish if not outright slow site loading time.
Image ads
If the image ads are a small size then they won’t have much of an impact on your site loading time. If the image ads are LARGE and have a large file size also then your site will definitely load slowly. The more of these large image ads you add to your site. The slower your site will get.
What to do: decrease the file size of the images or make the image’s dimensions smaller. The smaller the image and less file size it takes will lead to a boost in site loading time.
Flash ads
Flash ads generally have a larger file size than regular image ads. The more flash ads you add the more files your visitor has to download on each visit. If there are more than one flash ad then your visitor may have a really sluggish experience because Adobe Flash Player has been known to crash browsers and lead to performance issues.
What to do: keep Flash content to a minimal, better if there was no Flash content.