On the other hand, I was thinking that in general terms we shouldn't struggle too much when it comes to retrieve data from a REST API, and I like REST by the way :) 

Since I like REST too, in my humble opinion, that could be seen as kind of code smell in a sense that the API's architecture is not very well designed? 

So, here are alternative scenarios (for PHP) to solve the problem of a slow REST API that requires too many operations to fetch the data:

1. Write your own API with REST improving the architecture of the original one -- I tried to do something similar to fpl at [Data Driven Test Development Demo](https://github.com/programarivm/data-driven-test-development-demo)

2. Write your own API with GraphSQL -- [GraphQL Demo for Symfony 4](https://github.com/programarivm/graphql-demo-symfony4) 

In both cases, we're assuming that the API interface can be changed, and also it is okay to "scrap" and store the data on our side.