Why my Azure web app work faster on dev slot than on production slot?

Question

We have a Azure App Service, where users can view their device-related GPS positions on google map. When users click a button, a XML file is created on Azure cloud and transmitted to client browser for map rendering.

Our app service has two slots: production slot and dev slot. Both slots are based on the same source code. They run under the same Azure App Service Plan and use the same database.

However, recently I found that for the same job, production slot works much slower than the dev slot. For example, it takes dev slot 3.69 seconds to transmit a 10.8MB XML file from server to client; it takes production slot 3.2 minutes to do the same thing. Restarting production slot did not help.

I believe both slots worked similar before. However, I don't have proof of it. Any help will be appreciated. Thanks

Answer 1

It turned out that Azure Environment Variable "WEBSITE_LOCAL_CACHE_OPTION" is not in effect although its value ("Always") is CORRECT. Simply restarting app service does not help.

I have to change its value from "Always" to "Disabled", restart app service;

then change it back to "Always" and restart app service again in order to make this cache option in effect again.

This is really weird. I am pretty sure this environment variable worked before. Another Azure Tech support helped me to set it in 2024. We never change it since then.

Answer 2

Hi! Although the slots share an App Service Plan, they can have different configuration, traffic, caches, networking, and worker state. Compare slot-specific settings and test both slot hostnames directly, bypassing Front Door or other proxies. Use Application Insights dependency and request timing to identify whether the three minutes are spent generating the XML or transmitting it. A 10.8 MB XML response should also be compressed, streamed, or moved to Blob Storage rather than buffered by the web app.