Performance of toLocaleString is slow, due to lack of caching.

Problem

The performance cost of toLocaleString becomes a significant bottleneck when formatting hundreds of dates, e.g. for presentation in a list or grid view. In our case, which is a react app that continously polls for data, this is resulting in significant jank when scrolling.

Root cause

Profiling the code, the root cause seem to be that Luxon instantiates a new Intl.DateTimeFormat every time toLocaleString is called, which has a relatively high cost.

Suggested solution

Since there will only ever be a very limited number of formats, those format instances should be cached after creation. This reduces the runtime of toLocaleString cost by a factor of 10.

[icambron]

Released in 1.4.6. I got an 85x speedup on toLocaleString()

[pandameister]

awesome, thanks a bunch!