Jeffrey Dean recently gave a talk “Building Software Systems at Google and Lessons Learned” at Stanford (video). One of his slides was the following list of numbers:

L1 cache reference 0.5 ns
Branch mispredict 5 ns
L2 cache reference 7 ns
Mutex lock/unlock 25 ns
Main memory reference 100 ns
Compress 1K bytes w/ cheap algorithm 3,000 ns
Send 2K bytes over 1 Gbps network 20,000 ns
Read 1 MB sequentially from memory 250,000 ns
Round trip within same datacenter 500,000 ns
Disk seek 10,000,000 ns
Read 1 MB sequentially from disk 20,000,000 ns
Send packet CA → Netherlands → CA 150,000,000 ns

Everyone who wants to design high-performance, scalable systems should memorize these numbers. There are many, many lessons to be learned.