Extremely slow first response after idle period (~11 s) on GET /boxes/:id — cold start?

[Viper780]

Describe the bug

A single GET request to the OpenSenseMap API after a longer period without traffic (hours) takes about 11 seconds to respond (first byte). Subsequent requests answer in < 0.2 s (tested locally right after the slow one). This suggests a cold/idle path on the backend (e.g. MongoDB connection pool / instance cold start), not a network issue: DNS resolves in ~3 ms, TLS handshake < 50 ms, and the response is HTTP 200 once it arrives.

This is a problem for real-world clients with conservative timeouts:

• Home Assistant's opensensemap integration (opensensemap_api 0.4.1, hard-coded aiohttp.ClientTimeout(total=5)) polls 3 stations; on every cold cycle all 3 requests time out at the same time (Unable to fetch data from openSenseMap:), so sensor values go stale for up to a polling interval.
• Same pattern would affect any script/device with a timeout of 5–10 s.

To Reproduce

# from Vienna (Wien), 2026-09-05, api.opensensemap.org (128.176.196.25), DNS ~3 ms
# after ≥ a few hours without requests:
curl -m 60 -o /dev/null -w '%{time_total}\n' https://api.opensensemap.org/boxes/5e49643cb131a1001bb79466
# → 11.15  (first request)
curl -m 60 -o /dev/null -w '%{time_total}\n' https://api.opensensemap.org/boxes/5e49643cb131a1001bb79466
# → 0.09   (immediately again)

Observed

• time_namelookup: 0.003 s (DNS fine)
• time_connect / TLS: < 0.05 s (TCP/TLS fine)
• Time to first byte: ~11 s on first request, < 0.2 s afterwards
• HTTP 200 when it eventually answers
• Idle threshold: 45 s idle → 0.11 s; 6 min idle → 0.17 s; ~4.5 h idle → 11.15 s
  → the slow path only kicks in after longer idle periods (hours), not minutes
• Same behavior observed for /boxes list requests (bbox) before
• No errors/5xx; just a very slow first response

Expected

A single box document (small JSON, existing dataset) should answer in well under 1 s even after idle. If this is a connection-pool / instance cold-start issue, options would be:

• keepalive / pool warm-up on the backend, or
• an always-warm instance / health probe to avoid the cold path, or
• documenting the response-time/idle behavior for integrators.

Context

• API version reported by api.opensensemap.org: v11.3.0
• Related historical issues: Windspeed #311 (2019, slow responses under load), API does not return json when it asked to do #325 (2021, 502/429 bursts from a Vienna user)
• Related general issue about MongoDB high latency on first request: https://community.fly.io/t/mongodb-high-latency-on-first-request/21121
• This is about the API backend itself, not the frontend/web app.