Where are the comments?!! Data and performance trade-offs on ft.com Rhys Evans @wheresrhys

Probably the fastest pink news website in the world

Why is ft.com so fast? ● Early on we artificially slowed the site down for some users, and demonstrated that poor perf would cost money ● This helped get agreement from stakeholders, all the way up to the board that speed is important. ● So able to avoid pitfalls such as ○ Tag managers and other 3rd party nonsense ○ “My thing absolutely must go on the front page” ○ “The design uses this font-weight, and you will respect the design’s authority!” ● There is no secret recipe - we’re just empowered to apply best practice and to say no when it matters

Lazy-loading, one of our favourite tricks Deferring the loading of less important features ● Initial page load only includes the most important things - content, navigation, minimal amount of styles ● Use IntersectionObserver (thank you polyfill.io) to load images, comments, ads etc. as the user scrolls down the page later Bandwidth is not taken up by less important downloads, so the main content loads super fast ...so everyone’s happy?

“Where are the comments?!!” - about 8 users a week

● Why are comments so slow? ● Because we lazy load what is already a slow third party service… but it makes the site quicker ● Yes, very interesting... so let's load them earlier. It’s what the users want How to respond to this? Stakeholder conversation:

DATA

Able to easily collect data on how many users scroll as far as the comments … it’s really not very many We have a great in-house data collection tool - spoor

But sometimes data is not your friend I’ve mislaid the graph... ...the data told us but this is roughly how long... … it often took to load comments. ...sorry… what were we talking about again?

● But breaking perf rules is a slippery slope ● Making one thing faster will probably slow something else down Whatever you do, hold the line! So we had to do something

Different users care about different features Build a strong, performant baseline for the core features every user needs - being able to read a news story For secondary features identify which users care about them and personalise their experience accordingly … DATA is your friend again The answer - performance isn’t one size fits all

Step 1- partition users into groups - active comment user interacted with comments in the last month - passive comment user seen comments in the last month - everyone else fingers in their ears Data to identify the user’s cohort is stored in localstorage and used to decorate analytics Users in each cohort

Step 2 - Verify the cohorts make sense Small proportion of our users are in the ‘active’ cohort... … but they account for a large proportion of comment views

For active users, we started loading comments as soon as possible after the dom finished loading. Step 3 - apply discretionary performance tweaks Big improvement for active users... Without penalising the bandwidth of unengaged users % of comment views loaded within 2s

3 key facts: 1. We still lazy load for most users 2. Load within 2s 75% of the time for users who care 3. Underlying service 75th percentile response time is 7s

3 key facts: 1. We still lazy load for most users 2. Load within 2s 75% of the time for users who care 3. Underlying service 75th percentile response time is 7s 2s 2s 2s 2s 7s 7s 7s 7s

Impact Massive decrease in frequency of people asking me to improve comments

Happy customers joining the debate

TL;DO (too long; drifted off) ● Even when empowered to prioritise performance, you may still be asked to implement “improvements” that harm performance overall ● Explain what the trade-off is - you often can’t make a part faster without slowing down the whole - and try to say no to anything that will harm performance of your core product ● Use data to ○ put the demand for performance-harming features in perspective ○ learn more about users who are perceiving performance to be slow ● Personalise feature delivery based on what you learn about your users

Any comments?