Simon Willison’s Weblog

How we think about Threads’ iOS performance (via) This article by Dave LaMacchia and Jason Patterson provides an incredibly deep insight into what effective performance engineering looks like for an app with 100s of millions of users.

I always like hearing about custom performance metrics with their own acronyms. Here we are introduced to %FIRE - the portion of people who experience a frustrating image-render experience (based on how long an image takes to load after the user scrolls it into the viewport), TTNC (time-to-network content) measuring time from app launch to fresh content visible in the feed and cPSR (creation-publish success rate) for how often a user manages to post content that they started to create.

This article introduced me to the concept of a boundary test, described like this:

A boundary test is one where we measure extreme ends of a boundary to learn what the effect is. In our case, we introduced a slight bit of latency when a small percentage of our users would navigate to a user profile, to the conversion view for a post, or to their activity feed.

This latency would allow us to extrapolate what the effect would be if we similarly improved how we delivered content to those views.

[...]

We learned that iOS users don’t tolerate a lot of latency. The more we added, the less often they would launch the app and the less time they would stay in it. With the smallest latency injection, the impact was small or negligible for some views, but the largest injections had negative effects across the board. People would read fewer posts, post less often themselves, and in general interact less with the app. Remember, we weren’t injecting latency into the core feed, either; just into the profile, permalink, and activity.

There's a whole lot more in there, including details of their custom internal performance logger (SLATE, the “Systemic LATEncy” logger) and several case studies of surprising performance improvements made with the assistance of their metrics and tools, plus some closing notes on how Swift concurrency is being adopted throughout Meta.