前幾天 Hacker News 上看到「When network is faster than browser cache (2020) (simonhearne.com)」這則 2020 的文章，原文在「When Network is Faster than Cache」這邊，講 Firefox 在 2017 年導入了一個特別的設計，除了會在 cache 裡面抓資料以外，也會到網路上拉看看，有機會從網路上抓到的資料會比 cache 先得到，這個功能叫做 RCWN (Race Cache With Network)：「Enable RCWN」。
開頭就先提到了有人回報 Firefox 上的 RCWN 似乎沒有明顯效果：「Tune RCWN racing parameters (and make them pref-able)」。
On my OSX box I'm seeing us race more than we probably need to:
Total network request count: 5574
Cache won count 938
Net won count 13That's racing almost 16% of the time, but only winning 1.3% of the time. We should probably back off on racing a bit in this case, at least.
16% 的 request 決定 RCWN 兩邊打，但裡面只有 1.3% 是 network 比 cache 快。
不過作者決定試著再多找看看有沒有什麼方向可以確認，但測了很多項目都找不到哪個因素跟 cache retrieval time 有直接相關，反而在看看 Chromium 時發現 Chromium 是透過限制連線數量，降低 I/O 造成的問題：
It turns out that Chrome actively throttles requests, including those to cached resources, to reduce I/O contention. This generally improves performance, but will mean that pages with a large number of cached resources will see a slower retrieval time for each resource.
看起來就是個簡單粗暴的 workaround...