Does Ad sense affect SEO?

Participating in Google AdSense does not affect your site's rank in Google search results and will not affect the search results we deliver. Google believes strongly in freedom of expression and therefore offers broad access to content across the web.

Does site speed make a big difference in Google adsense cpc?

Yes, Adsense can hurt seriously SEO. But is not something specific from Adsense, but any ad network. Unless you place Ads in a very respectful manner with massive clear indications about what is ads and what not (for example, ads between content are very bad because they interrupt the user experience and Google checks for this with ease).
Why Google has implemented an auto ads feature if it pops the ads everywhere and interrupts this on purpose? Who knows, but 100% of the times I have set up an auto-ads campaign after adsense approved site, I've dropped in rankings no matter what. Still, it's worth for me a drop if I can monetize the rest of the traffic. It's just like the day you decide to go jogging with a weighted vest