The physics ceiling on what your system can do in a millisecond.
Latency Numbers Every Engineer Should Know: the physics ceiling on what your system can do in a...
make the invisible limits visible
Edge cache → freshness lag.
Latency at each layer is a physical constant. L1 cache: ~1 ns. Main memory: ~100 ns. SSD random read: ~150 µs. Round trip same datacenter: ~500 µs. Cross-continent: ~150 ms. These numbers don't change much, and they bound what your design can ever achieve.
Within a millisecond you can: do ~1M memory loads, ~6 SSD random reads, or ~2 same-datacenter network round trips. That's it. If you need more, batch or parallelize.
Cross-region travel is dominated by light-speed: SF→London ~70 ms one way at best. No software change makes this faster — only colocation, anycast, or moving compute.
Disk seeks (~10 ms) on spinning drives are 100x worse than SSDs. If a design depends on millisecond p99 with HDDs, it's dead on arrival.
Design a feed read with strict 50ms p99.