I recently switched to nginx from Apache webserver and noticed improved SERP. Google’s Webmastertools show you why:
Google gives your website a measured contingent of time its bot crawls your site. nginx’ reponse time is like 4 times faster than Apache, thus Google can crawl more content.
Besides that, Google (mainly AdSense) lowers your website’s rating when the webserver’s response time is too slow.
Unfortunately I cannot bring this thesis to proof but I’m pretty sure server response time is very important in Google’s algorithms. I’d love to see my visitors sharing their experience!
Indexation is quite a straightforward issue, every site has an indexation cap based on a number of factors including: PageRank, trust, site / server speed, duplicate content.