Experiencing random 15 seconds delays


I’m experiencing a slow response every now and then. The delay, if it happens (+/- 10% of the requests), seems to be exactly 15 seconds. Web server logging shows that the request only reaches the web server after those 15 seconds, after which it responds instantly.

I used cURL to see where the problem might be. A TLS connection to Cloudflade is setup instantly, an http GET request is sent, and then silence for 15 seconds:

SSL connection using TLSv1.2 / ECDHE-ECDSA-AES128-GCM-SHA256
ALPN, server accepted to use http/1.1
Server certificate:
subject: C=US; ST=CA; L=San Francisco; O=Cloudflare, Inc.; CN=sni.cloudflaressl.com
start date: Jul 30 00:00:00 2020 GMT
expire date: Jul 30 12:00:00 2021 GMT
subjectAltName: jasperhammink.com matched
issuer: C=US; O=Cloudflare, Inc.; CN=Cloudflare Inc ECC CA-3
SSL certificate verify ok.
Host: jasperhammink.com
User-Agent: curl/7.45.0
Accept: /
Connection: close

[Delay 15 s]
HTTP/1.1 200 OK
Date: Sat, 27 Feb 2021 10:27:29 GMT
Content-Type: text/html
Transfer-Encoding: chunked
Connection: close

The delay is right before the 200 response. After the delay I see the request popping up in my access log. If I bypass Cloudflare, there are no delays.

Any ideas on what could be causing this or how I could troubleshoot it? A TCP timeout and retransmission perhaps? I’m on a free plan, so no rate limiting. Thanks for your help!

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