This is generally true.
But it also depends on what type of database you have (OLTP vs. Data Warehouse), and what type of disk you have (JBOD internal, RAID-5, SAN, NAS, etc.
Unless you do a lot of table scans (typical in data warehouse databases) you are unlikely to see any difference, especially if you have a lot of memory allocated to your bufferpools.
Even with table scans, if you have SAN or NAS disk, you are unlikely to see any difference.
System temporary tablespaces should always use SMS because DB2 can create temporary tables much faster on SMS than on DMS. When creating tables on DMS, DB2 spends more time looking for the optimum contiguous space to place the table.
M. A. Feldman
IBM Certified DBA on DB2 for Linux, UNIX, and Windows
IBM Certified DBA on DB2 for z/OS and OS/390