JSON api best way to make it fast

I'm currently working on an Json API for procedral generation of galaxies and I have a lot of calculation, "dices roll", the thing are actually running with PHP, but I'm looking for something faster, another language, organization, ...

The outpu Json is organize like a tree, a graph, the root is the galaxy itself, the galaxy contain stars clusters, which contain stars systems, which contain planets

galaxy -> clusters -> systems -> planets

each level has it own properties, but planets can be only generated with the star data (for generating temperature, atmosphere, ...)

I have in mind to make dummy generation, with empty data and only have the seed for each level, and create endpoint to generate data for each level except for planet (I can only generate type and name without knowing the star)

but with that I can't get a "full generation" of a galaxy.

the API will be use in web, or in game engine like UE4

I'm open to every advice