Cognos 10.2.1 DQM开启后 对性能提升有多大帮助？

It's highly depending on the application itself and often times it's affected by how the reports were written. In general, DQM is a Java query engine, so it offers JDBC connectivity to many new vendors. It also has 64bit memory address space means it can handle larger result sets. The following Redbook will give more detail on some of the DQM capabilities:

http://www.redbooks.ibm.com/abstracts/sg248121.html

Hope it helps.