There is a memory overhead associated with guarded heap allocations. The time overhead is only applicable when creating or destroying them.

For encrypted data there is virtually no memory overhead but a guarded allocated has to be created to hold the decrypted data when it is needed.

The root encryption key is stored within a special container [0, 1] that has some small but constant CPU overhead as it is continuously mutating the memory associated with the structure in the background. This overhead can be tweaked (or even virtually eliminated) by changing the interval between successive cycles.

[0] https://spacetime.dev/encrypting-secrets-in-memory

[1] https://spacetime.dev/memory-retention-attacks