> What if you need to call multiple external APIs at once with complex json?

A few years ago, I had a PHP project that had grown by accretion from taking a single complex input and triggering 2-3 external endpoints to eventually making calls to about 15 sequentially. Processing a single submission went from taking 5-10 seconds to over five minutes.

This was readily solved by moving to ReactPHP (https://reactphp.org/), which implements async via event loops. I was able to reduce the 15 sequential external API calls to four sequential loops (which was the minimum number due to path dependencies in the sequence of operations). That reduced the five minutes back to an average of 20-30 seconds for the complete process.

It wasn't using true multithreading, but in a situation where most of the time was just waiting for responses from remote servers, an event loop solution is usually more than sufficient.