Comment by dtournemille

Yes, a flatter URL structure will likely improve SEO. I've seen this with our SEO consultants on our site. But URL structure is one aspect. Backlinks and quality content are probably more important. And mobile. If your mobile experience isn't up to par, you could be penalized. And speed. Is your site fast? If not, ya, could be penalized, too. You should have a look at your Google Webmaster Tools account first.