Saving the query in localStorage won't make it any faster. If anything it will make it slower. The performance issue the article mentions is that deciding whic

Saving the query in localStorage won't make it any faster. If anything it will make it slower. The performance issue the article mentions is that deciding which format to use in JavaScript breaks preload scanning and essentially serializes resource requests that would otherwise be parallel. Doing the check itself is fast. It's just decoding a very small image in memory. Storing the result of that check persistently just adds more cost.