Therefore, delay performance optimization until the latest possible moment. Concentrate on good design that won't slow you down, so that you don't waste time that you later can use to optimize your system.
Then use a profiler to find the performance bottlenecks. They aren't where you think they are (even if you are an expert)! And they are easiest to find in a well designed system.
Then optimize the bottleneck you found. It's likely that the 90/10 rule applies: you can improve performance by 90% by optimizing just 10% of your code. That's again the easier the more well designed (decoupled) the code is.