Why can't the result of make be shared across computers

I have a legacy work project which consists of 8000+ files. It includes the source code but also libraries so it can run independently.

However, when I run the make command the first time it takes roughly 4 hours, as opposed to executing the command the second time, after code additions, which is a lot faster and usually takes 5-10 minutes.
My first question is why is it faster the second time around?

Secondly, how come when the entire directory is transferred (after make has been built) to a different computer that the make command still takes 4 hours to complete and not 5-10 minutes?

The main purpose of GNU make is to compile code and do so without recompiling code without changes. In fact, man make states right in the first sentence of the description:

> The purpose of the make utility is to determine automatically which pieces of a large program need to be recompiled, and issue the commands to recompile them.

and a little bit further down:

> make updates a target if it depends on prerequisite files that have been modified since the target was last modified, or if the target does not exist.

This pretty much explains the speedup on second and following compilations. Not everything is compiled but only the parts of the project affected by the code changes.

When you copy your source code (and build artifacts) to another computer, the modification times will most likely be off. This will cause make to recompile everything. To prevent this from happening, preserve the time stamps when copying.