When is a cryptosystem or its algorithm (like RSA) considered efficient? When is a cryptosystem (like RSA) or its algorithm for keygeneration, encryption and decryption of messages considered efficient? Is there some bound in complexity which splits both efficient and not efficient cryptosystems or algorithms of cryptosystems?
From a theoretical perspective in cryptography, a cryptosystem is efficient when the execution time of the algorithm run by the legitimate user grows as a polynomial of a security parameter $k$, while the execution time for the best algorithm an adversary can run to break the system grows faster than any polynomial of $k$ (some change that to: exponentially with $k$, thus giving a different, less common definition).

This definition does not require to specify what $k$ is, and it's readily proven that changing $k$ to a polynomial in $k$ does not change the outcome of the above test. But often, $k$ is the size of something in bit, like the key, or the signature. In symmetric crypto, it is typically the size of the secret key.

From a practical perspective, the line is not clear. Make it: whatever is acceptable in the context.