Sure Structure Text is more efficient than FBD. See manual Library Object Style Guide, chapter 6 Programming. Therefore it is recommended to use ST for programming inner code of CM or FB types, which are used multiply times in control application. Languages like FBD, SFC, Diagrams likely have some overhead compared to ST.
There are some tricks how to decrease memory consumption, e.g. using addressing function blocks by FBname.Parameter . Note shortcoming is CBM Search Tool can't find connections by FBname.Parameter in the control logic. Define string variables like string[length]. Use attribute By_ref in your library types for in/out parameters. That makes sense for strings and structured data types but not for single variables. Avoid using too much code Tabs, since each Tab is equal to one POU. Minimize string operations.