Why use HKDF for key derivation even it's not time demanding? KDF such as scrypt is memory (cpu) demanding so that the attackers cannot quickly decrypt the key.

Why use HKDF?  What's the application of it?
If the attacker knows the input space of HKDF, and since the procedure of HKDF is public, what's the extra benefit of using it?
HKDF is not used to generate a secret key from a public input or a low-entropy password. It's used to derive a key from some secret input (e.g., a DH result) with high entropy.