From QaFoo:

The current status of the performance optimizations is roughly
summarized at:

https://github.com/ezsystems/research/blob/master/performance/docs/optimizing_status.txt

Verdict: Performance already almost increased by factor 8.

There are several related code changes in the ezp-next/performance
branch.

Most important changes:

• Caching languages and content types
• Optimizing identifier quoting
• Adding missing indexes
• Small query optimizations

By now MySQL is clearly the bottle neck, without any other remaining low
hanging fruits. And every query, which is executed more then once,
properly uses indexes. Reducing indexes to the minimal required set will
probably help with the insert performance.

Considerations:

Added indexes needs to be reflected in legacy as well so we should not add indexes without also trying to remove some, there are already duplicates, lets not have even more of that.