git/Documentation/config/fastimport.txt

fastimport.unpackLimit::
	If the number of objects imported by linkgit:git-fast-import[1]
	is below this limit, then the objects will be unpacked into
	loose object files.  However, if the number of imported objects
	equals or exceeds this limit, then the pack will be stored as a
	pack.  Storing the pack from a fast-import can make the import
	operation complete faster, especially on slow filesystems.  If
	not set, the value of `transfer.unpackLimit` is used instead.