VitalSentinel Documentation

This reference guide explains all performance metric thresholds used by VitalSentinel and Google's Core Web Vitals assessment.

Core Web Vitals

Core Web Vitals are the key metrics that affect Google search ranking and user experience.

LCP (Largest Contentful Paint)

Measures loading performance - how quickly the main content becomes visible.

Rating	Threshold	Description
Good	≤ 2.5s	Fast loading experience
Needs Improvement	2.5s - 4.0s	Could be improved
Poor	> 4.0s	Slow loading, needs optimization

What affects LCP:

Server response time
Resource load times
Render-blocking resources
Image optimization

CLS (Cumulative Layout Shift)

Measures visual stability - how much the page layout shifts unexpectedly.

Rating	Threshold	Description
Good	≤ 0.1	Stable layout
Needs Improvement	0.1 - 0.25	Some layout shifts
Poor	> 0.25	Unstable, frustrating experience

What affects CLS:

Images without dimensions
Dynamically injected content
Web fonts causing FOIT/FOUT
Ads and embeds without reserved space

INP (Interaction to Next Paint)

Measures responsiveness - how quickly the page responds to user interactions.

Rating	Threshold	Description
Good	≤ 200ms	Responsive interactions
Needs Improvement	200ms - 500ms	Noticeable delay
Poor	> 500ms	Sluggish, unresponsive

What affects INP:

Long JavaScript tasks
Main thread blocking
Heavy event handlers
Third-party scripts

Additional Metrics

TTFB (Time to First Byte)

Measures server responsiveness - time until the first byte of the response.

Rating	Threshold	Description
Good	≤ 800ms	Fast server response
Needs Improvement	800ms - 1800ms	Server could be faster
Poor	> 1800ms	Slow server response

What affects TTFB:

Server processing time
Database queries
Network latency
CDN configuration

FCP (First Contentful Paint)

Measures perceived load speed - time until any content is visible.

Rating	Threshold	Description
Good	≤ 1.8s	Quick first paint
Needs Improvement	1.8s - 3.0s	Delayed first content
Poor	> 3.0s	Very slow first paint

What affects FCP:

Render-blocking resources
Font loading
Critical CSS delivery
Server response time

Lighthouse Scores

Synthetic monitoring uses Lighthouse which provides scores from 0-100.

Performance Score

Score	Rating
90-100	Good (green)
50-89	Needs Improvement (orange)
0-49	Poor (red)

Score composition:

First Contentful Paint (10%)
Speed Index (10%)
Largest Contentful Paint (25%)
Total Blocking Time (30%)
Cumulative Layout Shift (25%)

Accessibility Score

Score	Rating
90-100	Good
50-89	Needs Improvement
0-49	Poor

Best Practices Score

Score	Rating
90-100	Good
50-89	Needs Improvement
0-49	Poor

SEO Score

Score	Rating
90-100	Good
50-89	Needs Improvement
0-49	Poor

TBT (Total Blocking Time)

Used in Lighthouse as a proxy for INP (since lab tests can't measure real interactions).

Rating	Threshold	Description
Good	≤ 200ms	Responsive
Needs Improvement	200ms - 600ms	Some blocking
Poor	> 600ms	Heavy blocking

Speed Index

Measures how quickly content is visually displayed.

Rating	Threshold	Description
Good	≤ 3.4s	Quick visual progression
Needs Improvement	3.4s - 5.8s	Moderate speed
Poor	> 5.8s	Slow visual loading

Mobile vs Desktop

Thresholds are the same for mobile and desktop, but:

Mobile typically has higher values due to slower devices and networks
Desktop usually performs better with faster hardware
Both matter for search ranking depending on the search device

CrUX Assessment

Google uses the 75th percentile (P75) to assess your site:

If P75 is "Good" → Pass
If P75 is "Poor" → Fail
If P75 is in between → Needs Improvement

This means 75% of your users must have a "Good" experience to pass.

Recommended Alert Thresholds

Conservative (Fewer Alerts)

Metric	Warning	Critical
LCP	> 3.5s	> 5.0s
CLS	> 0.15	> 0.3
INP	> 300ms	> 600ms

Aggressive (Catch Issues Early)

Metric	Warning	Critical
LCP	> 2.5s	> 4.0s
CLS	> 0.1	> 0.25
INP	> 200ms	> 500ms

RUM Monitoring

Track real user metrics

CrUX Monitoring

Google's assessment data

Setting Up Alerts

Configure performance alerts

Performance Thresholds