The web API fetches data from mod sites and the wiki on demand, and caches the fetched data for subsequent requests. The implementation is simple, but has disadvantages:

• A broken remote site may affect SMAPI's web services (e.g. mods.smapi.io breaks if the wiki does).
• All data is updated at the same rate, so can't have slower updates for sites with stricter rate limits.
• API calls may take a while or time out when querying a slow site or many mods.

Consider prefetching data for known mods with a background service instead. The web API/UI should indicate if the data being shown is stale (e.g. because the site is down), and still support fetching data on demand for unknown mods.

This is mostly done; further work will be done as needed separately.