Is it common to include Login / Sign Up actions on an app's splash screen? Or is it more efficient to first have a Splash screen, and then follow with a dedicated Log In / Sign Up page?
Avoid a splash screen, they are more for show than they are useful. The login/sign up page should be the first thing the user see's (if they need to log in to access content, i.e Facebook), if the user can access the site without logging in, then do not force them - have the option else where. So have a dedicated login page.
I would suggest skipping the splash screen, as this will only delay the user. The answer to your login/signup question depends on the requirements of your application. Here are the various ways it could be done.
If authentication is required to access any content on the application, because it will not function without it, then have a dedicated login/signup as your first screen.
If login is only required for some of your applications features, then I would drop the user directly into your application. When a user attempts to use a feature where authentication is required, then ask them to login/signup.
SIGN-UP should be first and big, because you want to acquire new customers, Sign-in is for regulars, used to the interface (upper-right corner), they know exactly where it is and most probable a cookie is giving them access anyway.
A great example of this is Digital Ocean homepage