Exchange 2010 no longer has the setting. Exchange 2003 IMAP4 client use the streaming store (.stm file) for reading and writing mailbox data that is stored as MIME content. If you use the Move Mailbox functionality in Exchange System Manager or Active Directory Users and Computers to move a mailbox that is accessed by an IMAP4 client, the data in the moved mailbox is promoted into the Extensible Storage Engine store (.edb file), and the content is converted from MIME content to MAPI content. When the IMAP4 client logs on, exact message sizes are calculated, and MAPI-to-MIME conversion of the mailbox data occurs in memory and on the disk of the server.
Fast message retrieval improves performance of the MAPI-to-MIME conversion.

Exchange 2010 no longer use .stm file.

More information

Enable Fast Message Retrieval for IMAP4
http://technet.microsoft.com/en-us/library/cc164353(EXCHG.80).aspx

Understanding the Exchange 2010 Store
http://technet.microsoft.com/en-us/library/bb331958.aspx

Answer from Jerome Xiong

Support for fast message retrieval

Fetching the body of an e-mail message was also possible in Exchange server 2003 SP2, but has been improved further with Exchange Server 2007. For example, you no longer need to select Mark for download or click Get the rest of this message, instead this will happen automatically in the background and thereby make the end-user experience even better.

Link: http://social.technet.microsoft.com/Forums/en-US/exchange2010/thread/d599ae36-0b0e-429c-b6a9-f7c8121764e7/
Link: http://technet.microsoft.com/en-us/library/aa996406(EXCHG.65).aspx
Link: http://geekswithblogs.net/BWCA/archive/2010/01/25/imap4-fast-message-retrieval.aspx