If you look at most of Google’s architecture, tools and systems from a high enough level you realize that it is all a designed around the concerns of latency (and expected failure, but this post is about latency). Latency in networks, in disk reads and writes, from main memory to CPU, you name it. A handy chart that keeps me awake at night:

Relative Data Access Latencies, Fastest to Slowest
CPU Registers (1)
L1 Cache (1-2)
L2 Cache (6-10)
Main memory (25-50)
—- don’t cross this line, don’t go off mother board! —-
Hard drive (1e7)
LAN (1e7-1e8)
Floppy, CD-ROM (1e9)
WAN (1e9-2e9)