Suggestion for if you want to continue to use this contractor, figure out what your metrics are for a "successful" solution (delay less than x seconds, allows for x users to be added, something like that), then build incentives into the contract for delivering in the timeframe you want vs steps beyond. or, if you want to use the stick rather than the carrot, penalties for going over what you want your timeline to be.
They'll either bail because it's not possible or it'll help you figure out your priority of requests for the agile sprints. And if you can't get anybody to sign on, you'll realize it's not a reasonable request.
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