Solving Drupal Performance and Scalability Issues

We often talk about Drupal Performance in relation to setting up Varnish, Boost, writing VCL files - I am not going to tell you any of this.
What we rarely talk about is what to do when performance issues are already there.

Often we end up making random guesses at what the issue might be or try to just "optimise all-the-things" which can be a never-ending (costly) task.
To solve performance issues we need to know what is actually happening, so we can choose to apply effort where it makes a difference.
Here I will share a data based approach for identifying and solving performance issues, along with tips on the low hanging fruit we so often miss, common pitfalls and best practices.

This session is for anyone that ever had a page on their site load slowly, outages due to performance issues or a site that needed mountains of hardware to run at all.