It takes about half as much (both startup time and memory) if you don't enable code completion.
If you are desperate about saving memory, disable all plugins.
If you are really desperate, build Code::Blocks yourself, defining NDEBUG and undefining cbDEBUG.
Also, you can patch the sources:
a) disable the resource preloader (found in manager.cpp)
b) do a global search for ": public BlockAllocated" and replace the second template parameter with 1 in all cases.
This may possibly save you around 2-3 MB of RAM total, but will make the application significantly slower, too.