Memorize these orders of magnitude:
- L1 cache: ns
- L2 cache: ns
- RAM access: ns
- SSD read: μs ( ns)
- HDD seek: ms ( ns)
- Same datacenter round-trip: μs
- Cross-continent round-trip: ms
The takeaway: Memory is x faster than SSD. SSD is x faster than HDD. Network calls add milliseconds.
When you say "add a cache," you're saying "move data from μs to ns."