Once upon a time, it was joked that nobody would ever need more than 640K of memory.¹ Today, some people wonder of what use ever increasing multi-core processors will provide. But given current technology, it seems a performance wall exists around 48 cores according to MIT researchers. The problem centers around concurrent access to the same memory. At certain levels of concurrency, processes are using different portions of memory and there aren't any conflicts with running in parallel. However, as the number of cores increases, the probability of overlapping needs to access the same overhead and management code (and thus memory) increases to the point that contention becomes a performance bottleneck.

¹Wired Magazine uncovered that it's merely an urban legend that Bill Gates is attributed to have uttered those words.