These days, whenever i read "headet only" i immediately get scared about compile times. Does using this library make compilation expensive in the way that eg protobuf or nlohmann_json do?

I'm biased but in my experience, no, not at all.

I don't use the amalgamated version, though (that only exists for this standalone version) and the library overall is significantly smaller than either of those and doesn't drag in nearly as many standard library headers.