HTML whizz's and a digital/interactive designer wanted

Dependency on an externally hosted item to be fetched before the page can be rendered.

Banner ads slow the rendering speed and therefore decrease the perceived speed of the site.

well, no - you would host them, as you would also need them to feature clicktag code so you could monitor number of impressions/clicks etc. if the data size is limited and the banners well implemented it shouldn't affect perceived speed - the banner might load slower than the rest of the page at worst.

anyway, i'm no expert - but a tiny decrease in perceived speed is no reason not to do something. especially if it annoys Hippy.