> The only real cost is in memory footprint

There are also load and store barriers which add work when accessing objects from the heap. In many cases, adding work in the parallel path is good if it allows you to avoid single-threaded sections, but not in all cases. Single-threaded programs with a lot of reads can be pretty significantly impacted by barriers,

https://rodrigo-bruno.github.io/mentoring/77998-Carlos-Gonca...

The Parallel GC is still useful sometimes!