Remember that premature optimization is the root of all evil. Write clear, correct code first, then if you have performance problems, use a profiler to look for specific areas to improve. Don't waste time optimizing code that's not affecting the overall performance in a meaningful way.
Measure, measure, measure. Performance can often be counterintuitive, and changes over time. What's faster now might not be in the future, and can depend on your use case. Make sure any optimizations you make are actually improving, not hurting performance, and that the change is worthwhile.