You can also point PSORT_DBTEMP to ramdisks. This seemed to help us.

/ramdisk/psort1/
/ramdisk/psort2/
/ramdisk/psort3/
/ramdisk/psort4/
....

then
PSORT_DBTEMP=/ramdisk/psort1:/ramdisk/psort2:/ramdisk/psort3/:/ramdisk/psort4/
....

You just make sure that the ramdisks are created when the machine comes
back on line before informix comes backup.

We use more than one directory because I thought and others concurred
that Informix would sort more in parallel with more directories. Of
course if you are running 10 use the high DS_NONPDQ_QUERY_MEM setting
as eriegger recommended.