typical IO latency:

L1 cache: ~2 cpu cycle
L2 cache: ~20 cpu cycle
DDR3: ~10 ns
DDR2: ~15 ns
RDMA: < 1 us
infiniband: ~ 1 us
flash

• read: ~50 us
• write: ~ 15 - 80 us

eithernet: ~0.2 ms
HDD:

• Average latency ~4ms
• Random read seek ~10ms
• Random write seek ~10ms

Internet: ~5-500 ms


Another source from http://blog.mixu.net/2011/02/01/understanding-the-node-js-event-loop/

Reference:
http://en.wikipedia.org/wiki/CAS_latency
Scalable High Performance Main Memory System Using PCM Technology"
http://www.fusionio.com/platforms/iodrive2/
http://www.intel.com/design/flash/nand/mainstream/index.htm