Intentionally slow decryption

What you need is a slow KDF which processes a stored or memorized password and converts it into a key for decryption. The actual decryption process can proceed rapidly, as long as the key-derivation takes a long time. There are a number of algorithms that can do this, such as PBKDF2, Argon2, and scrypt. If each stored record contains its own password that must be first put through a strong KDF before the record can be decrypted, then you will be able to enforce a configurable delay to access.

I strongly suggest, however, that you contact a lawyer rather than trying to implement a technical solution yourself. There may be better legal alternatives that would allow you to get away with not storing any data. Never assume that your own interpretation of the law is the only possible interpretation.