what is the best way to import an module if it is not needed in all time [on hold]

what is the best way to import an module if it is not needed in all time
[on hold]

what is the best way to import an module if it is not needed in all time?
Will the import slow down the application with the import in the head?
what is the best way to import an module if it is not needed in all time?
Should i import the module in the head of the file without a condition or
should i import it with a condition?
Will the import slow down the application with the import in the head?