Cash bond is easiest legally - a 2 party transaction - no Bondsman/Surety intermediary. A family member or equivalent, on behalf of the defendant/felon, puts up the total bail amount, in cash, to the Court as assurance that that the defendant/felon returns from bail. The Court holds the money until bail has been complied with. The family member will get money back, minus fees, if the terms of bail are met (showing back up and not violating bail terms). If the defendant/ felon does not return - the whole amount is forfeited to the Court.
Surety bond is more complicated - a 3 party transaction. Really the only option at $6M. The team of a Bondsman, backed by a Surety Company, enters into a binding legal contract with the Court, on behalf of defendant/felon, assuring $6M could be liquidated by the Bondsman/Surety, for the benefit of the Court, if defendant/Felon disappears. The Court is not in the business of tracking down money from fleeing felons. The Court gets the $6M. Underneath the Surety Bond - a Family member pays a percentage of the bail in cash (amounts vary) and must secure the non cash remainder with collateral. The Bondsman/Surety can liquidate the family assets if the defendant/felon flees in order to offset the amount they gambled away on the Surety bond.
This is why bond will takes days in this case. Financial security for $6M will not be easy.