fix: slow intersections on IAzureLayerStatefulProviderProps type

[Weffe]

I'm part of the Kusto team (Azure Data Explorer) and we stumbled upon a recent issue where the type checker would heavily slow down after a change a member on our team did to tighten up types by referencing IAzureLayerStatefulProviderProps in our code.

This caused type checking (npm run tsc) to slow down 10-20x the normal time. Usually for our large project it would take 30s to type check but after the introduction of IAzureLayerStatefulProviderProps some devs saw a slowdown to 90s and without caching upwards of 5mins-10mins. We found that by removing any reference to IAzureLayerStatefulProviderProps and deleting its import then TypeScript would resume back to normal performance levels.

I found that if we replace the intersections (&) with interface + extends then the issue is fixed and TypeScript isn't affected by the slowdown. The assumption here is that TS takes a lot longer to squash all the intersected types down to a single type because each intersecting type has complex shapes on its own defined properties. At least with an interface TS does some extra perf tricks and is why they recommend interfaces over intersections on their performance wiki

You can try out a repro of the issue here in this Codesandbox

In a terminal, run npm run typecheck to get the before stats and then manually edit the types.d.ts file to include the new changes and run again to see the after stats. Here's a screenshot of the stats I saw. A rough 3x increase in type checking speed which matches our experience.

[yulinscottkang]

Looks good, thank you!