Event schema

Fields the query expects

Field	Type	Why it exists
timestamp_utc	Timestamp	Task completion time.
route_template	Utf8	Stable page route.
release	Utf8	Frontend build identifier.
duration_ms	Float64	Observed main-thread task duration.
interaction_related	Boolean	Whether the task overlapped a measured interaction.

DataFusion SQL

Copy the query

sql

SELECT
  route_template,
  release,
  COUNT(*) AS observed_tasks,
  SUM(CASE WHEN duration_ms > 50 THEN 1 ELSE 0 END) AS long_tasks,
  SUM(CASE WHEN duration_ms > 50 THEN duration_ms - 50 ELSE 0 END)
    AS blocked_time_ms,
  100.0 * SUM(CASE WHEN duration_ms > 50 THEN 1 ELSE 0 END)
    / NULLIF(COUNT(*), 0) AS long_task_rate_pct
FROM browser_task_events
WHERE timestamp_utc >= now() - INTERVAL '7 days'
GROUP BY route_template, release
ORDER BY blocked_time_ms DESC;

This read-only query is planned and executed against an empty typed table with Apache DataFusion 45.2.0. We review the synthetic sample output separately. Check field types, thresholds, and counting rules against your own data. Read the testing methodology.

Query result

Blocked main-thread time by route

The editor creates more than twice the blocked time despite fewer observed tasks.

Compare a numeric result field

/editor

52,180 ms

comparison 7 ms

/dashboard

23,860 ms

comparison 3 ms

route_template	release	observed_tasks	long_tasks	blocked_time_ms	long_task_rate_pct
/editor	web-103	9,200	644	52,180	7
/dashboard	web-103	15,400	462	23,860	3

Synthetic example output. Run the query against your own event schema and thresholds before using it for operational decisions.

Blocked main-thread time by route: static chart of synthetic blocked_time_ms values from the Calculate browser long-task rate by route example result — Download this SVG chart of the sample results for an article, runbook, or design review. Please credit Telemetry.

Reproduce the example

Download the sample data

The JSON bundle includes the event schema with field types, illustrative input rows, exact SQL, expected output, review notes, and engine version. The CSV contains the displayed result.

Sample JSON Expected output CSV Static SVG chart

How the SQL works

1The 50 ms boundary follows the browser long-task convention.
2Blocked time subtracts the allowable first 50 ms so severity is additive.
3Group by release to compare long tasks before and after a deployment.

Edge cases to check

Browser support and sampling rates must be documented.
Background-tab work can be misleading.
Long tasks indicate blocking but do not identify the responsible function without complementary profiling.

Recommended dashboard

Bars: blocked_time_ms by route
Trend: long_task_rate_pct by release
Table: interaction-related long tasks

Alert guidance

Review sustained blocked-time regressions after a release; page only when tied to a critical workflow.

Read alert setup

Set up the events this query needs

Related instrumentation and guides

Frontend performance monitoring Core Web Vitals recipe Code profiling guide

Continue the analysis

Run it on your events

Create a table, adapt the fields, and save the result

Start free, send structured events, and use the query result as a chart, shared dashboard widget, or alert input.

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Calculate browser long-task rate by route