Gwern notes an observed 10-15% decrease in traffic while A/B-testing banner ads, at https://www.gwern.net/Ads; and those ads were quite discreet. That article contains a summary of some other ad-related results, all showing a similar decline in traffic in other places (including in an audio context), which weakly suggests that however fast your ads load, you're still going to drop >5% of your users simply by the fact of having ads at all.