Double-keyed Caching: How Browser Cache Partitioning Changed the Web (via) Addy Osmani provides a clear explanation of how browser cache partitioning has changed the landscape of web optimization tricks.
Prior to 2020, linking to resources on a shared CDN could provide a performance boost as the user's browser might have already cached that asset from visiting a previous site.
This opened up privacy attacks, where a malicious site could use the presence of cached assets (based on how long they take to load) to reveal details of sites the user had previously visited.
Browsers now maintain a separate cache-per-origin. This has had less of an impact than I expected: Chrome's numbers show just a 3.6% increase in overall cache miss rate and 4% increase in bytes loaded from the network.
The most interesting implication here relates to domain strategy: hosting different aspects of a service on different subdomains now incurs additional cache-related performance costs compared to keeping everything under the same domain.
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