In T5137#139066, @werner wrote:

Note that you actually run 30 independent processes with gpg 1.4 but with gpg-agent there is just one process to handle the private key operations (decrypt). To utilize more cores you need to setup several GNUPGHOME with the same private keys.

In T5137#139064, @gniibe wrote:

I think that it is not gpg-agent but pinentry which causes millions of futex syscall errors.
For interactive use case, pinentry may be the point of contention.
I might be wrong if your key is not protected by passphrase.

If possible, please try adding arguments for gpg invocation: --pinentry-mode loopback --passphrase-file YOUR_FILE_FOR_PASSPHRASE
This can avoid the invocation of pinentry entirely.