Like, you can't name many of the ways a big, 
     high-traffic site might slow down, because 
     the internal tools websites you work on don't 
     need caching or proxies or load balancing

I feel like it would be tough to spend any amount of time developing web apps, even if they're just internal tools, without hitting a basic performance issue like an n+1 query situation where you accidentally write a page with 500 queries or something. But maybe that's me... maybe devs are better than me these days. I hope so =)

There are of course innumerable network related reasons an app could slow down and we wouldn't expect a software dev to dive into them... I'd expect them to just be able to do some ultra basic stuff to rule out a network issue and zero in on the application stack.