That was my first thought too, but OP mentions they already explored fernet, which was not performant enough. Fernet applies AES 128 in CBC mode, with a SHA256 HMAC message authentication code so aes_encrypt/decrypt will probably be comparable in performance (depending on key strength). (AES is hardware accelerated on most x86 cpus so it should already be very fast)

But frankly: any cryptographic operation will take cpu, so for me it is not clear what the client expects.

PS. +1 for Unity Catalog