Android Gradle Implementation vs CompileOnly Performance

The docs mention that implementation provides significant build time improvements over compile/api. What about compileOnly?

My use case is a multi-module (sorry I don't like Gradle's multi-project terminology) project, where I have an Android app, and multiple libraries that the app depends on (implementation). Some of the libraries also depend on one another. Should I use implementation or compileOnly when declaring dependencies in the library modules? My app module will be using implementation to depend on those artifacts, so I don't need them to be transitive through the library modules.

The api configuration should be used for dependencies that are exported to external modules (transitive dependency). Vice-Versa implementation configuration should be used for dependencies that are internal to the component (not transitive dependency).

implementation vs compileOnly:

There is no similarity in their job, compileOnly is

a configuration inherited from java-plugin
required at compile time
also not included in the runtime classpath or exposed to dependent projects.

So compileOnly doesn't replace the implementation configuration job e.g:

implementation 'com.android.support:appcompat-v7:25.1.0' // can't use compileOnly here
testCompile 'junit:junit:4.12'

compile "com.google.dagger:dagger:2.8" // can't use here also
annotationProcessor "com.google.dagger:dagger-compiler:2.8" // can't use here also
compileOnly 'javax.annotation:jsr250-api:1.0' // we can use compileOnly here because it's required on run time only.

Since your case is a "multi-module", you have to use the api configuration, until you reach the final module it's better to use implementation.

Following graph describe those configurations:

Performance?

I think api requires more memory because gradle will snapshot every class in that transitive module, vice versa implementation is a preferred configuration because (as mentioned above) it's used for its own internal implementations.

In Android Gradle plugin 3.0 the compile keyword has been now deprecated in favour of implementation and api.

api: you leak the interface of this module through your own interface, meaning exactly the same as the old compile dependency
implementation: you only use this module internally and does not leak it through your interface

Read more about api vs implementation here and here

compileOnly dependencies function similarly to provided, allowing you to declare non-transitive dependencies used only at compilation time.

Compile-only dependencies address a number of use cases, including:

Dependencies required at compile time but never required at runtime, such as source-only annotations or annotation processors;
Dependencies required at compile time but required at runtime only when using certain features, a.k.a. optional dependencies(using it);
Dependencies whose API is required at compile time but whose implementation is to be provided by a consuming library, application or runtime environment.

Compile-only dependencies are distinctly different than regular compile dependencies. They are not included on the runtime classpath and they are non-transitive, meaning they are not included in dependent projects.