Dear Anonymous A,
No, it does not necessarily mean you were not successful.
I'll break this down into three scenarios:
+ Was this an informal/informational interview:
Since McKinsey rarely does telephone interviews for recruiting purposes (video conference is the default when in-person sessions are not possible), I expect this could likely have been just an informal chat, or pre-interview assessment. In that case, there is no standard time to hearing back. However, I would expect in such a case to hear back sooner rather than later. If two days have lapsed, it does not augur well, I'm afraid.
Caveat: If this session took place on a Thursday or a Friday (i.e., heading into the weekend), or before a major holiday, then the two-day delay might be as a consequence of decision personnel being unavailable outside business hours.
+ Was this a First Round interview:
McKinsey's delivery and feedback time-frames for formal First Round interviews are fairly standardised worldwide. Everyone (and I know quite a few) I've ever known to interview with McKinsey received feedback (irrespective of being successful or not) within 24 hours. Most do so within three hours of the interview's conclusion.
Caveat: If this session took place late on the last working day of the week, then the two-day delay might be as a consequence of decision personnel being unavailable outside business hours. But this would be rare.
Since the overwhelming majority of decisions are communicated within this ~24-hour time-frame, I really cannot think why there has been a delay in your case given, of course, that this was a formal First Round interview.
+ Was this a Second/Final/Decision Round interview:
For final interviews, standards tend to be less uniform across the board. Partners who conduct these interviews have far more pressing scheduling, locational, and client priorities to strictly abide to quick turnarounds.
I've known people to receive their feedback within an hour, some within two days, and some within just under a week. I've never heard of cases going beyond 5-6 days. Therefore, if this was a Decision Round interview, I would say the two-day delay is an indication of nothing.
I realise I've been able to assuage the anxiety you must naturally be feeling, but I hope that in providing this information, you can at least make more informed assumptions.