> ... memory and cpu performance have improved at completely different rates.

This is overly simplified. To a first approximation, bandwidth has kept track with CPU performance, and main memory latency is basically unchanged. My 1985 Amiga had 125ns main-memory latency, though the processor itself saw 250ns latency - current main memory latencies are in the 50-100ns range. Caches are what 'fix' this discrepancy.

You would need to clarify how manual memory management relates to this... (cache placement/control? copying GCs causing caching issues? something else?)