The biggest speedup I obtained was from reducing the number of "includes" and "requires". Each one of these is a physical disk access, and whilst a good caching and optimizing module can help, I still realized huge parse time decreases by getting rid of as many of these as possible.
Far more significant than worrying about relatively inconsequential things like single vs double quotes for example.
Next on my list would be optimizing your database - using indexes and making sure your queries use them. Benchmark long running queries to see if they can be rewritten. Never assume that even a relatively simple query can't be improved.
Turning on the query cache helps, too, if you are using MySQL, and making sure the database configuaration parameters are optimized for your particular situation.