You seem to only be considering the case where the only logins you need to deal with are invalid logins. A busy and successful service will see the vast majority of logins being for legitimate, known users where the bcrypt check must consume CPU time. You have to design the system to be able to handle the workload from both good and bad logins without revealing information about a bad login to an attacker. Anything other than going through the same motions every time will leak information in ways you don't expect.
Also, there is no reason to implement an extensive backlog in the first place. It's far better for a system at 105% load to drop one in ten attempts than to have an infinitely growing queue.