I don’t think you need be concerned about time. The things to think about are whether to use include or require and whether or not to use once
The way I look at it is
include - when I want something but it isn’t a critical FAIL if it doesn’t get included
require - when it is absolutely necessary and a FAIL would result in major breakage
once - if there is a chance there might be “can’t redefine” errors. I find this is generally a safe default except when I intentionally want to get something more than once
I find this a very good technique because it is very easy to change a required filename, make changes and when satisfied keep the changes.
With lengthy scripts it is difficult not to create problems when making changes and drastically simplifies debugging.
Edit:
There will be an extremely slight difference in loading time usually about a couple of milliseconds. Compared to debugging time I would not be concerned over the additional time taken to load the required script.
As chorn stated, every bit of code will take up some time. But as long as it’s like .00056 ms. Does it make much difference?
The thing you will need to be careful about using require_once is getting the path and file name right. One wrong or misspelled name, typo or case mismatch, and the script will die right there.