I make websites fast. Permanently.
Call me when you've hit a wall. You can't find what's making your site slow, or you found it and don't know what to do about it. I find the cause and fix it together with your developers.
Some of the teams I've worked with
Making web performance work
You've worked through the Lighthouse list and the site is still slow. I see this on most projects. The suggestions were right. The order was wrong. A preloaded hero image gains you nothing while 500ms of scripts block the main thread in front of it.
I find out what has to happen first on your site, and we start there. More than 300 sites so far.
I measure before I touch anything
First I install CoreDash, a RUM tool that shows how your real visitors experience your site, per page, per device and down to the element causing the delay. So we fix what hurts your users.
We fix it, together
You don't get a PDF and an invoice. I work inside your team and prefer your developers make the changes, so they understand why it works. Where the skills aren't there yet, I write the code myself. Either way I stay until the Core Web Vitals are fixed.
Your team owns it when I leave
I add performance budgets to your CI/CD pipeline, train your developers and leave CoreDash running, so a bad deploy gets caught before Google notices. You can keep me around, but you won't need to.
Loop Earplugs
"Mobile got about 800ms faster and checkout conversion went up 7%. It paid for itself pretty much straight away."
Nathan Ziarczyk
Results
15 marketplaces, featured on web.dev
I reorganized how third-party scripts load across 15 Adevinta marketplaces. Google published it as a web.dev case study.
Fixed in one 60-minute session
Their team had been working on layout shifts and the Largest Contentful Paint for months. We found it in one session, I sent the plan the same day and the field data showed the result within a week.
Performance partner for an agency
Happy Horizon brings me in when a client site needs more than their own developers can handle. We work together across their whole portfolio.
What clients say
Swift
"The layout shift on our checkout is gone. CLS went from 0.4 to 0.02, on mobile and on desktop."
Louis Breton
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
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."
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
Alza
"Arjen taught our team how he works. We now find and fix performance problems ourselves."
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
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
The tool behind the work
CoreDash: Real User Monitoring for Core Web Vitals
I built CoreDash because I needed it. On every project I have to see, while it's happening, which element or script is making a page slow or a click lag. After years of digging that out by hand, I turned it into a tool. It stays running when I leave, and it has saved more than a few of my clients' necks.
Real time, down to the cause
See what your visitors experience right now, and the exact element or script behind every slow page.
Works with your AI
Ask your AI assistant why checkout got slower yesterday. CoreDash answers through MCP.
Data stays in the EU. No cookies, no IP addresses, no personal data, so no consent banner. And no limits on how many pages you track or how long you keep the data.
Hit a wall? Tell me about it.
Send me a message with your URL and what's going on. I'll take a look and tell you what I think is wrong and what it would take to fix it. No sales pitch. And if it turns out you don't need me, I'll tell you that too.
Mobile LCP is stuck around 3.8s. We went through the whole Lighthouse list and saw no improvement.
https://www.example.com/
Had a look. Your hero image is preloaded, but the consent banner script runs first and holds up the main thread. Fix that first, the rest of the list matters a lot less until then.
I'll ping your dev and we'll fix it together.