Performance Audit 2017: Beacon: ResourceTiming Compression

[nicjansma]

ResourceTiming is currently the most expensive plugin as far as JavaScript CPU time and beacon size for larger sites.

Depending on the site and device, compressing the ResourceTiming entries could take 20-100 milliseconds or more.

We should investigate ways of reducing this cost. Some ideas:

• Before compressing the entries, we must iterate over all of the frames in the page to gather all entries. We could use the PerformanceObserver in some (or all) frames to get notified of new entries, instead of crawling for them
• The optimizeTrie is the most expensive operation, where it iterates over each character in every URL. We could look into forming a non-perfect Trie by grouping every 10 characters together, or something

[bluesmoon]

For optimizeTrie, we could also just split on [./?&=] and use those as groups. Also, reversing the hostname should be built in.

We should also see if we can move the optimization over to a WebWorker and use postMessage to send the resulting string back to the main thread.

[nicjansma]

We have a fix in review (Akamai repo), which splits the URL at the path separator / instead of by every character in convertToTrie().

Summary of the benefits:

• These optimizations are more important on larger sites (i.e. 100+ URLs)
• On sites where the ResourceTiming optimization was taking > 100ms (on desktop CPUs), changing to splitting at / reduced CPU time to ~25ms at only a 4% growth in data (restiming) size.
• On sites taking 25-35ms, changing to / reduced CPU time to ~10ms at 2-3% data growth

[nicjansma]

Merged in as 7834819