IIS response time hight every 10-15 minutes for the same simple request

Issue

We have a performance issue with an AngularJS website hosted on IIS. This issue only affects our users connected via VPN (working from home).

The problem: regularly, a page that usually takes one or two seconds to load can take over 10 seconds.

This issue first appeared to be random, but we were able to reproduce it in a test environment and found out that the problem seems to arise on a very regular basis (every 10-15 minutes).

What we did: using a tool (ThousandEyes), we send every minute the same simple GET request via 12 clients to the Test server. We can see in the IIS logs that this request is processed in less than 50ms most of the time. However, every 15 minutes or so, the same request takes more than 5 seconds to process at least for 1 client. Example below: the calls done every minutes by client #1 takes more than 5 sec at 21:12, 21:13, 21:14, then 21:28, 21:29, then 21:45:

The graph below shows the mean response times for the 12 clients (peak every 10-15 minutes):

For both the test and the production environments, this issue only affect users connected via VPN (but not all the users connected via VPN are affected at the same time).

Any idea what can cause this behavior ?

All suggestions and questions are welcome.

Notes:

• Session State. InProcess. I tried Not Enabled and State Server but we still have the same results.
• Maximum Worker Process. 1. I tried 2, no change.
• Test server usage. As far as I can tell, nothing special happen every 15 minutes on the server (no special events).
• Test server configuration: 2 Xeon proc @2.6GHz, 8 GB RAM, 20 GB disk space, Windonws 2016.
• Test server load: almost nothing beside these 12 requests every minute from the 12 test clients.

Solution

This issue cost us a lot of time. We finally found out that a VPN server was misconfigured.

Rebuilding this server was the solution.

Answered By - Sylvain Rodrigue