Core Web Vitals case studies
What was slow, who fixed it and what the field data showed afterwards. Most of the fixes were built by the client's own developers. I found the problems, put them in order and checked the fixes.
Some of the teams I've worked with
Adevinta: the Fotocasa case study on web.dev
Four of Adevinta's fifteen marketplaces: Marktplaats, Subito, Fotocasa and Kijiji. On Fotocasa a filter click re-rendered the whole search page. It became a case study on web.dev: together with other work, contact and phone leads went up 27%.
Prijsvrij
Most layout shifts on this holiday booking site happened on soft navigations in the booking flow. LCP went from 2.6 to 1.8 s.
gmbapi
Their new Next.js site was fast on desktop and slow on mobile. Fixed during the call.
Voices
Three sites in one and a blog that scored well in Lighthouse but not for real visitors. Still passing four years after the audit.
Who What Wear
A product gallery built entirely in JavaScript, and ads that pushed the text around. CLS on mobile went from 0.30 to 0.
Loop Earplugs
Mobile INP on their Shopify store got about 800 ms faster.
Nina.care
A fast-growing childcare platform. Their developer and I built it together; I did the scripts, the chat widget and the image loading. CLS went to 0.
Speak
A year after the training their developers rebuilt a client's webshop. LCP went from orange to green in one step.
"Thanks to the concepts and suggestions from the training, our page speed and user experience have improved significantly."
"Arjen can explain the most complicated performance problems, and you can tell he loves this stuff."
"After the training we finally understood why our INP was failing, and which interactions we had to make cheaper."
Happy Horizon brings me in by name when one of their enterprise clients needs performance work.
What clients say
Miro
"Our dashboards were doing way too much work on the main thread. CPU usage went down 25%, and our users noticed it right away."
Suzanne Beek
Alza
"Arjen taught our team how he works. We now find and fix performance problems ourselves."
Zalando
"We've had audits before, and they all gave us a long list of problems. This one told us what to fix first, and why."
DPG Media
"He found problems in our component library that we had been living with for two years. Within a few days you could see the difference."
Ashkan Malekloo
Voices.com
"Arjen played a key role in fixing our Core Web Vitals. He got our critical profile pages back into the green in PageSpeed. What I value most is his hands-on attitude: he doesn't just find the bottlenecks, he works directly with our development team until the fixes are actually live."
Jayme Cousins
Rituals
"We used to break performance every other sprint. Arjen put performance budgets in our pipeline and we haven't had a regression since."
Laura Vogels
Swift
"The layout shift on our checkout is gone. CLS went from 0.4 to 0.02, on mobile and on desktop."
Louis Breton
Send me your URL
Tell me what's bothering you and I'll say which of these cases it looks like, and what it would take. I reply within a day.
