The time for the adjudication of the I-526 petitions is based upon government administrative time delays and therefore the time delays are constantly changing , not consistent and therefore the adjudication time delays are not predictable. Therefore the sooner you file the petition the sooner you will receive your conditional permanent residency.
It is indeed possible that the processing could take another 21 months. However, it is just as likely that the backlog can be processed in a very short period of time and more recent filings can be processed more quickly. The lesson here is that no one can predict the timeline for processing. What we can do, as experienced EB-5 attorneys, is assist investors with preparing approvable petitions to avoid further delays by requests for evidence or denials.