I've seen the code written by a Python web-programmer that for a given binary matrix of teams and tournaments produced top 10 rivals for each team where rivals are defined as teams that have picked the same tournaments to participate as your team.

There are tens of thousands teams and thousands of tournaments and his code was taking two days to precalculate the results for every team.

Written with minimal understanding of how slow the memory is, the new code takes less than a minute. There was no point of optimizing it further, the algorithmic changes were enough.