There is a bug in your tests case.  You are really measuring the time it takes to write 30k requests into the cache.  Each Cache.put() is an async operation and you don't wait for the returned promise to resolve.  So when you start your cache.keys() you are still waiting for all those disk writes to run.

You probably want to do a Promise.all() on the cache.put() promises and then start the cache.keys().  That way you can isolate the time to read vs write.

This isolates the read time from the write time:

function fill(cache) {
  let list = [];
  for (let i = 0; i < 30000; ++i) {
    list.push(cache.put('https://example.com/probably-crash-' + i, new Response('ok')))
  }
  return Promise.all(list);
}

function read(cache) {
  let start = performance.now();
  cache.keys().then(keys => {
    let end = performance.now();
    console.log(end - start);
  })
}

caches.open('foo').then(cache => { fill(cache).then(_ => read(cache) ) })

Thanks for pointing that out! In this case the primary problem is the crash (as this is a very rare event, and async operation, the time is less relevant). 

I've published an updated test here: https://rawgit.com/ConradIrwin/a634359060cbbde0e90a30e5744aa1a1/raw/94c524dcc451f7104b5fdf8999a56cc70db86ff2/storage.html

It still reliably crashes with "Operation too large"

"crashes" - actual browser or renderer crash, or just an exception?

Just an exception, sorry for the confusing. It correctly returns

    query_cache_context->callback.Run(CACHE_STORAGE_ERROR_QUERY_TOO_LARGE,
                                      std::unique_ptr<QueryCacheResults>());

Which is surfaced as a javascript exception "Operation too large"

That error is an unfortunate hack to protect the browser from using too much memory. Until we rewrite CacheStorage on the browser side to stream responses back to the renderer, we simply limit the number of responses we process on the browser side so that it doesn't get too big. :[

Note that I'm referring to the size of the response headers being an issue, we *do* stream the response body bytes.

Thanks! What's the target issue? It's unfortunately hidden :(

Deleted: Screen Shot 2017-02-03 at 19.24.08.png 495 KB

	Screen Shot 2017-02-03 at 19.24.08.png 495 KB View Download

Same thing as I described in comment 6 basically.