I reviewed and/or committed numerous 8.0.x critical issues, as well as major issues which support resolving critical performance issues.

I also attended calls with maintainers, as well as a one-off meeting to discuss https://www.drupal.org/node/2364011 [meta] External caches mix up response formats on URLs where content negotiation is in use, and made myself availably in irc to quickly give feedback on questions about critical issues.

I began tracking every issue I was looking at, but only managed to keep this up for the first week, however some of the issues included:

https://www.drupal.org/node/2364011 [meta] External caches mix up response formats on URLs where content negotiation is in use
https://www.drupal.org/node/2473983 [meta] Evaluate Entity Field API Scalability
https://www.drupal.org/node/2471228 Optimize merging of attachments
https://www.drupal.org/node/2381505 Unserialize preloaded routes on the fly
https://www.drupal.org/node/2447803 Use FileCache for caching discovered annotations
https://www.drupal.org/node/2343035 Upgrade validator integration for Symfony versions 2.5+
https://www.drupal.org/node/2421479 Use a bootstrap.php file to load the most used classes in one go
https://www.drupal.org/node/2450897 Cache Field views row output
https://www.drupal.org/node/2466585 Decouple cache implementation from the renderer and expose as renderCache service
https://www.drupal.org/node/1867518 Leverage entityDisplay to provide fast rendering for fields
https://www.drupal.org/node/1923406 Use ASCII character set on alphanumeric fields so we can index all 255 characters
https://www.drupal.org/node/2454513 [meta-1] Make Drupal 8 work with SQLite
https://www.drupal.org/node/2472323 Move modal / dialog to query parameters
https://www.drupal.org/node/2416109 Validate configuration dependencies before importing configuration
https://www.drupal.org/node/2322949 Implement generic entity link view field handlers
https://www.drupal.org/node/2381277 Make Views use render caching
https://www.drupal.org/node/2351015 Link CSRF tokens can be hijacked when cached with insufficient contexts
https://www.drupal.org/node/2335661 Outbound path & route processors must specify cacheability metadata
https://www.drupal.org/node/2263569 Cache-enabled forms generate cached form data for every user on every request

As well as reviewing/commiting those issues, I also worked directly on the following two critical issues:

https://www.drupal.org/node/2488538 - profiling memory usage and initial patch to reduce it.
https://www.drupal.org/node/1744302 [meta] Resolve known performance regressions in Drupal 8

In both cases I profiled to get a better idea of the current state, then worked on patches to resolve those issues, i.e. https://www.drupal.org/node/2480811, https://www.drupal.org/node/2479529, https://www.drupal.org/node/2480943, https://www.drupal.org/node/2354889

Best,

Nat