"Stateful scaling is much much harder than stateless scaling. Making your application stateless is hitting the scaling jackpot, it becomes just running multiple instances and routing requests. Stateful scaling involves distributed transactions, syncing caches, data routing, ... and much more"
"O(N) time complexity analyses tend to assume that: you can ignore constant factors; instructions have the same duration; memory doesn't matter; and instruction execution dominates performance. But on modern systems memory DOES matter and memory access can easily dominate over instruction execution. The processor can execute 300 instructions in the time it takes for data to be retrieved from main memory (RAM)."