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Doesn't Varnish handle many times more concurrent clients than apache does, at significantly lower system load? It could just be pure optimization.

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It does. Apache is strictly one process per connection, varnish handles hundreds, thousands or even tens of thousands of connections with just one process so the overhead is minimal.

Apache has not strictly been one process per connection since 2.0. See the worker MPM. I would have chosen nginx, but Apache can be configured as a capable static file server.

Even then each thread is still one connection (according to the apache docs).

Except in the Event MPM

It is pure optimization - they said it gives them a performance boost and smooths things out.

In their "flat-file" setup though - they are deliberately configured so that, should varnish fail (which was one of their concerns) - they don't actually need it - they could just have haproxy immediateley start hitting the apache servers directly.

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