What is CLS by TTFB
CLS by TTFB compares Cumulative Layout Shift (CLS) to the Time to First Byte — the delay between a request and the first response from the server.
While TTFB is a backend metric, long delays can cause content to arrive in bursts, leading to unstable rendering and shifts once the page starts painting.
Healthy CLS by TTFB sample
In a healthy setup, CLS remains low across all TTFB values, meaning:
Your layout skeleton is in place before content loads.
Server delays don’t cause layout surprises.
The frontend handles slow responses gracefully.
This reflects strong coordination between backend timing and frontend layout.
Unhealthy CLS by TTFB sample
If high TTFB aligns with poor CLS:
Critical resources might be late, causing sudden layout jumps once content arrives.
Server-rendered pages may shift when hydration kicks in or fonts load late.
If CLS is bad even with fast TTFB:
That likely signals client-side instability and should be addressed independently of server speed.
Resolving unhealthy CLS by TTFB
Go-to action plan to resolve an unhealthy CLS by TTFB:
Ask Uxi to analyze your CLS by TTFB values and suggest improvements
Use Filters to focus on slow TTFB sessions with high shift values.
Simulate CLS of the suspected lens to see if fixing it will resolve the CLS by TTFB. If yes, this is where the resolution focus should be.
Use an automated CLS optimization tool like Navigation AI to improve your CLS by TTFB values
Once you’ve improved CLS, set an alert to be the first to know if it starts worsening again.
Try it yourself
Discover how your website performs with real user data.