Note that you can't use this mode to get the most out of a subscription - they say it's always charged as extra usage:

> Fast mode usage is billed directly to extra usage, even if you have remaining usage on your plan. This means fast mode tokens do not count against your plan’s included usage and are charged at the fast mode rate from the first token.

Although if you visit the Usage screen right now, there's a deal you can claim for $50 free extra usage this month.