Telephony applications that use DTMF can only provide twelve options at each state (0 though 9, * and #). Speech recognition can provide hundreds and even thousands of options at a state. Even though the speech recognition can handle hundreds of responses, it's not possible to list all of the responses to the user because of the slowness of speech playback.
DTMF menus play a game of "20 questions" with the user. A financial telephony application might first ask the user if they want to get their balance or transfer. If they transfer it then asks the user from what account, and then to which account, followed by how much. Each menu has a limited number of responses.
Speech recognition menus need to provide the same architecture so that novice users are guided by the menus and know what to do. As a result, novice users will maneuver through a speech enabled telephony application at about the same speed as one that provides only DTMF. However, experienced users will learn shortcuts.
Once the user has completed a task the slow way, a telephony application might give the user a hint and tell him how to accomplish the task quicker the next time. For example, the financial application might say, "Next time you call, you can just say, 'Transfer $500 from checkings to savings." The user will be able to bypass several menus the next time, saving him/her time.