Is there any difference in compiled execution speed in creating and placing reusable code in a library instead of just using modules for personal projects?
Unless your code includes "heavy" calculations the difference in performance will not be significant (assuming that the code is efficient, libraries should run faster).
The advantages of using a module is that it will be easier to debug and easier to update as needed.
Libraries should mostly be used to access features which are not accessible directly from Basic4ppc.