To turn off optimization for a specific search, the last command in your search criteria should be
limits.conf has a switch that you can and should set to disabled in 6.5.0:
[search_optimization] enabled = <bool>
If you are asking about the search assistant (compact vs. full mode), it is in Account Settings > Search > Search Assistant. See Help building searches in the Search Manual for more information.
If you are asking about syntax highlighting, you can disable it in the same account settings. See Help reading searches in the Search Manual for more information.
The Search Manual also includes two topics about optimizing searches (not what you asked about, but I am adding the info here for people who might stumble on this topic in the future):