Slower clients can likely just be subsidized by faster clients since I imagine the return would be higher and more consistent than traditional ads. It's already been done on a few sites, so you could look into what they did, but I forget where.
I feel like the potential for abuse would still be a better tradeoff than the current situation of ads that try to download malware or redirect you to phishing sites.
And what about abuse via advertisers, injection, cross-site, etc?