/scratch on a Cray is (last I used one) is a Lustre filesystem. It is the only file storage available to the compute nodes, which have no local storage of their own. No spinning disk, no SSD. So if the code is written such that it makes frequent small writes (e.g. it's peppered with print statements), the lustre nodes get hammered by all the compute nodes and become the bottleneck and they will eventually fall over.