What is TTFB by INP
TTFB by INP tracks Time to First Byte (TTFB) - based on the responsiveness of a session, as measured by Interaction to Next Paint (INP). This lens helps uncover whether poor frontend responsiveness is also correlated with backend delays.
It essentially flips the perspective: instead of asking "Does backend latency hurt interactivity?" it asks "When interactivity suffers, is backend speed also a factor?"
Healthy TTFB by INP sample
Should you worry
In a healthy TTFB by INP view, both fast and slow INP sessions should show green TTFB values. This means:
The backend is performing consistently, regardless of how smooth the frontend is.
Responsiveness issues, if present, are likely isolated to frontend bottlenecks (JavaScript, rendering, input delay).
In this case, your backend isn't the bottleneck — it’s doing its job and delivering the initial payload quickly across the board.
Unhealthy TTFB by INP sample
Problems show up when TTFB starts creeping up in sessions with poor INP, such as:
Slow INP, slow TTFB: Backend delays are likely contributing to the overall sluggish experience. The user waits to see the page and struggles to interact with it — a worst-case scenario for engagement.
Fast INP, slow TTFB: Rare but suggests that the frontend is efficient even after a slow start. This may mask backend issues that still hurt time-to-visibility and conversion.
Slow INP, fast TTFB: Tells you that responsiveness issues lie further down the pipeline — possibly caused by render-blocking scripts, layout shifts, or main-thread contention.
This lens is especially useful when diagnosing why "everything just feels slow" to users.
Resolving unhealthy TTFB by INP
Go-to action plan to resolve an unhealthy TTFB by INP:
Ask Uxi to analyze your TTFB by INP values and suggest improvements.
Use Filters to focus on slow INP sessions with high TTFB and trace the server response origins — are these isolated to certain templates, locales, or routes?
Simulate TTFB of the suspected lens to see if fixing it will resolve the TTFB by INP. If yes, this is where the resolution focus should be.
Use an automated optimization tool like Navigation AI to improve your TTFB by INP values.
Once you’ve improved TTFB, 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.