Add documentation on Performance Expectations

docs/performance-expectations.md

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+# Performance Expectations
+
+## CDN Response Times
+
+Altis Cloud serves [static files](./static-file-caching.md) and the [page cache](./page-caching.md) from the CDN. In cases when static files and pages are already cached on the CDN, response times for client requests are typically between 10 milliseconds and 30 milliseconds.
+
+Response times can depend on the quality of connection from the client. See [CDN](./cdn/README.md) for details on Edge Cache locations.
+
+## Origin Cache Response Times
+
+The origin cache (also called Batcache) is documented in [page cache](./page-caching.md). Response times for Batcache HIT requests are typically between 30 milliseconds and 60 milliseconds.
+
+## Origin Response Times
+
+Origin response times when PHP / WordPress dynamically generates a page can vary significantly depending on the site's implementation, including which plugins are used, the active WordPress theme, how many database queries are used, etc. The included [development tools](docs://dev-tools/) help with visibility of these statistics.
+
+We're provided the following guidelines for page generation time to help teams better understand the typical "base" response times, based on our real-world experience.
+
+These figures may not be illustrative of all projects, but can provide a guideline for optimisation.
+
+Page Generation Type | Average (milliseconds) | 90th Percentile (milliseconds)
+-- | -- | --
+New WordPress Install + `Twenty Twenty Two` theme homepage. | 150 | 450