That config's actual label is "Script Verification Parallelization" which is mainly used when parallel script verification is required like when a new block is included.
This hits hard a node in IBD during its script verification of newest blocks height that aren't part of "assumedvalid" blocks.

Default is good enough for most systems.
But if you think that your machine can't handle it, (e.g.: Whole PC is freezing starting 85~90% of IBD),
you can set it to a negative value to leave one plus that much threads.