Post-migration performance testing: verify speed and stability
A migration is a critical turning point. Without tangible proof that the new site delivers on its promises, it's best not to declare victory too soon.
- Systematically compare the new site's Core Web Vitals to those of the old one
- Test across multiple browsers and devices, not just Chrome desktop
- A drop in organic traffic of more than 20 to 30% signals a serious problem to address urgently
- A documented report turns the test into proof, not just an impression
Why test performance immediately after a migration
When you change infrastructure, platform, or host, elements that worked perfectly can suddenly slow down. These issues aren't just a matter of user experience: Core Web Vitals are official ranking factors, and even moderate degradation can lead to a drop in organic visibility within days.
The Core Web Vitals to measure
LCP
Time to display the largest element. Target: under 2.5 seconds.
INP
Response time to an interaction. Target: under 200 milliseconds.
CLS
Visual stability during loading. Target: under 0.1.
Measure these three metrics on the new site, and compare them to the old one if possible via the history in PageSpeed Insights or Lighthouse. Significant degradation in any of them signals a problem to fix before validating the migration.
Load time and benchmarking tools
Measure load time on at least ten important pages, from several locations and connection types. GTmetrix and PageSpeed Insights are the two most reliable tools for standardized measurements, while WebPageTest offers more granularity for advanced tests. Also test from several browsers: an excellent time on desktop can be mediocre on mobile over 4G.
Availability testing and cross-browser compatibility
A continuous monitoring tool (Uptimerobot, for example) instantly alerts you if the site becomes inaccessible. During the first 24 to 48 hours, stay attentive to alerts. Also check compatibility on Chrome, Firefox, Safari, and Edge: forms, conversion buttons, and animations must work everywhere, not just on the most widely used browser.
Check 301 redirects and organic traffic
Check that each redirect works, with no loops or excessive chains: each one adds 200 to 300 milliseconds, acceptable once but not in a cascade. Organic traffic should remain stable or grow slightly during the first week; a drop of more than 20 to 30% signals a serious problem (poor indexing, broken redirects, degraded Core Web Vitals) to diagnose via Google Search Console.
These checks are an integral part of every website design and redesign project: the migration is only validated once performance is proven.
Produce a documented test report
A test without documentation is wasted work. A simple report (URL, date, Core Web Vitals, load time, browsers tested, issues identified, planned actions) makes it possible to track progress and formally prove that the migration is a technical success. Rank each issue by criticality: a broken form gets fixed within 24 to 48 hours, a minor visual detail can wait.
Frequently asked questions
How long should you monitor after a migration?+
Intensive monitoring for the first 48 hours, then regular follow-up over two to three weeks to confirm stability.
Is a small drop in traffic normal?+
Yes, a variation of a few percent is common while Google reindexes. Beyond 20 to 30%, you need to investigate.
Which free tools are enough for these tests?+
PageSpeed Insights, GTmetrix, Google Search Console, and Uptimerobot cover the essentials with no extra budget.