create table as select VS create table; insert as select

I have inherited an application that populates a number of
temp.y tables using create table ... as select ...

This is taking roughly five to ten minutes to run

As this process hammers the database, I can only run benchmarks at night so
am asking here if anyone know if

create table ...; then insert into ... as select... ; would be faster.

or if anyone can suggest an alternative I may have missed.

I am happy to move code server side if need be.

TIA

Jacqui

I know these is not a lot of detail in the above - the system is under NDA
and I need to check with my employer before I give out any system details.

Before you ask it is not a big customer - just a very paranoid one :-)