[qwertox]

Speed, probably. Lua is much faster.

[nattaylor]

Yes, speed. 1,000,000+ QPS and 1,000s of ads to evaluate is common. At that scale it's very distributed, so replication is another challenge.

In a way, its a search problem where bid≈relevance and targeting_match≈recall, so I've seen Solr used here too.