SvelteKit Core Web Vitals
Field-data LCP, CLS, and INP tracking with regression alerts.
The SvelteKit-specific angle
SvelteKit's hydration model keeps INP low by default, but +page.server.ts load functions that pull large payloads hurt LCP. The measurement most teams miss is the streamed HTML delay.
What we watch on SvelteKit
- LCP regression after adding a slow API call to a shared +layout.server.ts
- CLS from element sizes computed post-hydration in Svelte transitions
- INP jumps on routes with heavy $state runes graphs
Universal core web vitals checks
Same on every stack — these run on your SvelteKit site out of the box:
- Largest Contentful Paint (LCP) — 75th percentile, tracked hourly
- Cumulative Layout Shift (CLS) — with the specific shifting element flagged
- Interaction to Next Paint (INP) — replaces FID for 2026 rankings
- Alerts fire when the 7-day rolling baseline degrades by >20%
Reference thresholds we alert against
How Lemwatch handles it on SvelteKit
Route-level CWV data with the load-function timing overlaid, so slow loaders get spotted before they hit p75 rankings.
Core Web Vitals on other stacks
SvelteKit Core Web Vitals — FAQs
Route-level CWV data with the load-function timing overlaid, so slow loaders get spotted before they hit p75 rankings.
SvelteKit's hydration model keeps INP low by default, but +page.server.ts load functions that pull large payloads hurt LCP. The measurement most teams miss is the streamed HTML delay.
No. Lemwatch is fully black-box — we monitor what visitors and Googlebot see. Add your domain, checks start within 60 seconds, and no plugin, SDK, or code change is required in your SvelteKit app.
Pro plans run 1-minute checks from multiple regions. When a regression trips a rule (e.g. p75 LCP up >20% for 3 consecutive windows, or cert expiry <7 days), the notification fires within the next check cycle — typically 60 seconds.
Yes. Add HTTP basic auth headers or a bearer token per site, and Lemwatch treats staging as an isolated site — flaky preview envs won't pollute your production status board.