In a networked server getting data from memory, the time required to access the data itself is negligible, so the real performance in mostly a factor of how much I/O a thread can sustain (Redis with heavy pipelining will handle at least 500k ops/sec in the same hardware it handles 100k ops/sec without pipelining). There is still the case of the load to be very biased towards, like, 5% of keys. But that 5% of keys are very very likely to be distributed across all processes.

However this is not true when you have a case with, like, 2/3 super hot keys that are requested a lot more than any other. But in this case what allows scalability is replication with many read-replicas.