num_workers=1 spawns a separate process to load the data (I double-checked in top when running  )
If you try to use more than one worker, my guess is they will get in each other’s way when trying to read/write from the file, I think that option is more for loading data from sources where it’s easy to have multiple simultaneous handles to it.
In any case I think disk i/o is likely to be the bottleneck in your case, so I woudn’t expect increasing the number of workers to increase performance much.