I haven't been using spinning disks for perf critical tasks for a looong time... but if I recall correctly, using multiple processes to access the data is usual

I haven't been using spinning disks for perf critical tasks for a looong time... but if I recall correctly, using multiple processes to access the data is usually counter-productive since the disk has to keep repositioning its read heads to serve the different processes reading from different positions. Ideally if the data is laid out optimally on the spinning disk, a single process reading the data would result in a mostly-sequential read with much less time wasted on read head repositioning seeks. In the odd case where the HDD throughput is greater than a single-threaded CPU processing for whatever reason (eg. you're using a slow language and complicated processing logic?), you can use one optimized process to just read the raw data, and distribute the CPU processing to some other worker pool.