Why is cache important?

Consider the following: even old RAM technology such as DDR2 can deliver throughput in the GBps and latencies around the several nanoseconds. On the other hand, reading from regular spinning media (hard disk drives) throughput peaks at 150 MBps in most cases, and latency is in the several hundred milliseconds.

Reading from cache is always faster than going to disk—waiting for the disk to move its head, finding the blocks of data requested, then sending it back to the controllers and onto the application.

One thing to keep in mind is that cache needs to be warmed up first; this is the process of allowing the system to determine which files are regularly being accessed and then moving that data to the cache. While it is warming up, requests are slower, as they first have to be fetched from disk.