On App Engine, what does optimization for reads mean?
"Optimize for reads, not writes" means that you should expect to see far more reads than writes, and so you should strive to make it as easy as possible to read your data, even if that might slow down the writes a little. Easy for the computer, that is, meaning that for example if you want to show names in all lowercase, you should lowercase them when they're written to the database rather than lowercasing them everytime you read them from the database. That's just an example but hopefully it makes things clear.
In the documentation for Google App Engine, it says that when designing data models for the datastore, you should "optimize for reads, not writes". What exactly does this mean? What is more 'expensive', CPU intensive or time consuming?