perf: disable subgraph passthrough #208
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This is another case discovered by the performance testing actually. And that is that it turns out that while Firefox and Safari seem to share their script cache with their fetch cache, they don't actually seem to do the converse in sharing the fetch cache with the module cache. As a result, when polyfilling a module graph, if any of those subgraph modules are directed to the native loader URL instead of using the blob version this results in a double fetch and causes a bandwidth slowdown.
By ensuring that in the polyfill case all subgraphs use the blob form as fetched, we avoid this slowdown. For normal module loads that aren't blobs we maintain the native registry.
A documentation note is included for polyfill mode that having some graphs as native and some being polyfilled will result in no instance sharing between these.