Is Craft Suitable for Large Public Body website
One of the biggest Craft projects I've worked on has around 25 sections, 10+ global sets,  250+ fields, and 12,000+ elements across 16 locales (although only 1 locale has launched atm). Craft itself (the CP) has not had any problems I can remember handling it so far, especially with the element index updates that have been introduced over the last few months. I know there must be people running sites much bigger than that too.

On the front-end, we definitely had some slow templates where we are looping over a lot of related elements and outputting assets that were generating hundreds of queries per page, but careful planning, template caching, and in some cases using plugin code to pre-fetch data outside of the craft.entries etc calls in templates helped a lot. You can suffer from N+1 problems with relations and images - as you can with any CMS/website.

I wrote a small plugin for storing template caches in Craft's data cache (in our case Redis), which I'll see if I can release; we stuck Varnish in front of everything except CP requests; use S3 &amp; CloudFront for serving assets; and while there's still room for improvement, we're now getting straight A's on webpagetest.org, so Craft certainly isn't just for small hobby-projects.