I’m afraid there is no option to ‘downgrade’ from AES-256 to AES-128. Actually, the performance is likely to be determined by data transfer speeds to and from disk rather than encryption speed.
Not that I don’t trust AxCrypt, but always ensure you have proper backups of all data – especially when we’re talking terabytes. The more you have, the more you stand to lose…
The idea with the ‘Advanced’ menu option is that normally you’d be ok with working from the context menu, so the ‘Advanced’ does indeed just open up the AxCrypt GUI – that’s way more advanced than the context menu operations though. That’s why we named it like that anyway.