If using SMB, it could be security related. See Samba speed 25% slower after update - #19 by sam_nazarko. But this should cause a small slowdown but not a slowdown to the extent you have described
If using SMB, it could be security related. See Samba speed 25% slower after update - #19 by sam_nazarko. But this should cause a small slowdown but not a slowdown to the extent you have described