I agree with the final comment; I think the DOJ would be well-served by filing something (and that should be pretty tractable, seeing the staggering scope of apparent misdeeds) to extradite SBF as quickly as possible, and then to file amended/superseded charges in the following 12-24 months.
That's likely true.
However, it may also be a more savvy move to let SBF and other execs think little is pending in order to gather more information. Other than reputation of the justice system (which already has a justified slow reputation; "The wheels of justice turn slowly, but they grind exceedingly fine."), this might be the greater consideration...
If they don't rapidly indict, it'll be interesting to see if SBF ever voluntarily returns to US territory before an indictment.