Following is the summary of Search Modes taken from Splunk Documentation Splexicon:
1.Fast mode speeds up searches by limiting the types of data returned by the search.
2.Verbose mode returns as much event information as possible, at the expense of slower search performance.
3.Smart mode, the default setting, toggles search behavior based on whether your search contains transforming commands. For transforming searches, it behaves like Fast mode. For searches without transforming commands, it behaves like Verbose mode.