There's a vast gulf between the fastest and slowest memory and storage:

• CPU registers: Under 1 nanosecond access time, less than 1 KB • Cache (L1/L2/L3): Few nanoseconds, from tens of KB to several MB • Main memory (RAM): 50-100 nanoseconds, tens to hundreds of GB • SSDs: 25,000-100,000 nanoseconds (25-100 µs), terabytes

Caching and tiered memory help bridge this gap. By keeping frequently-used data in faster memory, the CPU can access it quickly. When the CPU needs data that's not in cache, it has to wait much longer to fetch it from main memory. These delays can add up to significantly slow down processing.

However, cache memory is expensive and limited in size. Main memory offers more space at a lower cost per gigabyte, while SSDs provide huge storage capacity for even less.

The key is using each level of the hierarchy optimally. Effective caching strategies and smart allocation of data between tiers allows computers to take advantage of the speed of fast memory and the capacity of slower storage. It's a balancing act that has a big impact on system performance.