I'm Arjen. I make websites fast
Core Web Vitals consultant since 2009, for teams whose site fails on mobile while Lighthouse looks fine. I find the cause in the field data, your developers fix it, and CoreDash, the RUM tool I built, makes sure it stays fixed.
Some of the teams I've worked with
Seventeen years, more than 300 sites
Adevinta, eBay, Zalando, Nestlé, DPG Media, KPN, Harvard, Loop Earplugs and Who What Wear, among others.
Red on mobile, fine in Lighthouse.
Most of my week goes to sites like that. The p75 on mobile fails, the team has worked through the Lighthouse list, and nothing moved. I start in the field data, find the LCP element that's still late or the script that holds up the click, and write down the cause, the effect and the fix. Then I check every release against the field data until it holds.
You can hire me for an hour or for months. A power hour for one question, an audit for the whole site, a fix I build myself, consultancy inside your team, a masterclass for your developers, or work for your agency's clients. Prices are on the services page, and the short version is on the Core Web Vitals consultant home page.
What the fixes did
A case study on web.dev
Four marketplaces: Marktplaats, Subito, Fotocasa and Kijiji. On Fotocasa a filter click re-rendered the whole search page. With the state moved into the button, that click went from 440 to 64 ms in a throttled test. It became a case study on web.dev: together with other work, contact and phone leads went up 27%.
+7% checkout conversion
Mobile got about 800 ms faster. "It paid for itself pretty much straight away," says Nathan Ziarczyk.
Fixed in two sessions
Their team had already done a lot of work on LCP and layout shifts. What was left was the product gallery, built entirely in JavaScript. We made the images render in plain HTML first, and their developers gave the ad slots a fixed size. In CrUX, LCP on mobile went from 2.5 to 1.5 s and CLS from 0.30 to 0.
More on my clients page.
What clients say
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
Swift
"The layout shift on our checkout is gone. CLS went from 0.4 to 0.02, on mobile and on desktop."
Louis Breton
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."
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
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
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
I build the tools I need on a job
CoreDash
Real user monitoring that shows, for every LCP, INP and CLS event, which element or script caused it. Teams at Nestlé, eBay, DPG Media and Harvard use it. It has an MCP server, so an AI agent can read the same field data I do.
State of CWV
190,720 sites: the CMS, framework and scripts each one runs, and how it does on the Core Web Vitals in CrUX. A crawler on my desk adds new sites every night.
Trace Explorer
Drop a Chrome performance trace and read it: the waterfall, the Core Web Vitals and every event, each one explained in the Unofficial Chrome Trace Event Manual.
Core Web Vitals Visualizer
A Chrome extension with 10,000 users.
WP Core Web Vitals
A WordPress plugin for caching, images, scripts, styles, fonts and resource hints.
MarketingTracer
The SEO platform I founded in 2004. It has checked more than 125,000 sites.
Your team should not need me forever
Every job ends with your developers doing it themselves. Sometimes that's the whole job: two hours with your team on your own pages. I trained the developers at DPG Media, Speak, Loop and Jamie Oliver Group that way.
A year after his team's training, Sander Los of Speak mailed me: "Thanks to the concepts and suggestions from the training, our page speed and user experience have improved significantly." How a masterclass works.
On podcasts
Why real user data beats a Lighthouse score, and how lazy loading the wrong element slows a page down. Listen
Launching a fast and free personal page. Watch
With Andrew Armitage: what Core Web Vitals do to rankings, and why a slow site is sometimes a choice somebody made. Listen
From a dial-up modem to State of CWV
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1996: Erasmus University
I started out as a psychologist. At Erasmus University I researched the internet when it was just emerging: how people get information from it. That led me to build my first site, and I haven't looked back since.
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2003: technical SEO
As a technical SEO consultant I built tools to measure the content and quality of a site. Page speed kept pulling me in.
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2004: MarketingTracer
Those tools became MarketingTracer, the SEO platform I founded. It has checked more than 125,000 sites.
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mod_pagespeed
When Google built mod_pagespeed, the Apache module that rewrote pages to load faster, I was one of its testers and wrote code for it.
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2009: page speed, full time
Page speed became the job. In 2020 Google named the metrics the Core Web Vitals, and the job barely changed: find what the visitor waits for and get it out of the way.
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2022: CoreDash
I couldn't find RUM that was affordable and precise enough for client work, so I built it.
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2026: State of CWV
Live, and growing every night.
Short bio
Free to copy, as is or shortened. The photo too.
Arjen Karel is a Core Web Vitals consultant from Bergen op Zoom, the Netherlands, and the founder of CoreDash, a real user monitoring tool. He started out as a psychologist, researching how people get information from the early internet. Since 2009 he has made more than 300 sites faster, among them Adevinta's marketplaces, eBay, Nestlé and DPG Media. His work with Fotocasa became a case study on web.dev. He was one of the testers of Google's mod_pagespeed and built the Core Web Vitals Visualizer, a Chrome extension with 10,000 users.
Download the photo · Arjen Karel on LinkedIn · KRL Media, Bergen op Zoom
Three kids, a tent and a long walk
I'm a father of three. About four times a year I take the kids camping, which gives my wife some time to herself. On my own I hike, as far from a road as I can get. If you call and I sound out of breath, I'm probably in the Ardennes.
I also guide hikes, but only for friends. That's a broad term: if you want to be my friend, I'll happily take you on a hike anywhere in the world.
Want me to look at your site?
Send me the URL and what's bothering you. It goes to my own inbox and I reply within a day. If you already know what you need, the services page has the prices.
