If you look in Google to find for something like subfolders nesting vs SEO performance, you will probably find not too much useful information. How nested subfolders affect SEO performance? Searching in internet it is difficult to say definitely it is good or bad to have to many subfolders, that is “/” characters in URL path to a page.
However, in my opinion and what I can see in searching results, websites with URLs containing less nested subfolders perform better. There can be several factors responsible for it, but at least three of them seems to be obvious. High number of nested subfolders usually means bad user experience. Users have to click more times to reach a page on website. If you expect somebody to visit more pages on your website, the nesting level should be lower. The faster people can reach all your pages the more they will be able to visit. Searching engines treat seriously user experience, because better user experience usually means better navigation system.
There is as well some technical issue. With every step deeper into website the number of possible pages increases. It means, that with every next level searching engines have to process more content. And the grow in needed resources increases exponentially (more or less). This is a bad news for some searching engines. Depending on content, they can stop their job at level 3 or 4 (out of 9 for instance).
Another factor, rather badly affecting websites with multilevel nested subfolders architecture is simple, but not that obvious. The point is, that with every level deeper into your website the “distance” between new pages emerging on crawler’s horizon increases (in a standard pyramid architecture). This can be somehow compensated with some sophisticated internal link structure between pages. But, as such, hierarchical structure reveals more information about content relationship inside a website and between website’s pages, than complicated network of internal connections, which require additional resources on crawlers part to sort it out somehow. We could say, that the deeper into your site we go, the more isolated are your pages. This isolation, increasing with every nesting level, decreases probability for correct and full classification of your pages. Needless to say, it badly affects page position too.
However, there could be exception. Let’s say your website is about travel, and has 100,000 pages about hotels, or you have photo gallery with millions of images, or you have an online shop, with many subcategories. But, again, even in such situation try to keep navigation simple, to help users find quicker what they are looking for.