1. You're not clever.
2. Distributed systems rule #1: Do not distribute the system!. At some point of scale this is often unavoidable, but be aware of the complexity that comes with a distributed system.
3. Do not micro optimize.
4. Measure where your program spends time.
5. Optimization rule #1: Reduce I/O.
6. Should I use a distributed cache? Only if you are aware of the error cases and how to mitigate them!

7. A complex system that works is invariably found to have evolved from a simple system that worked. Gall's Law

Inspired by Rob Pike's 5 Rules of Programming.