That’s a very broad question! There’s lots of different aspects to “performance”. In my day-to-day work, I mostly focus on load-time performance. The best advice I can give there is to ship less code :)
Invest time in setting up a robust build pipeline. Consider using a tool like webpack or rollup to minimize the size of your generated JS and CSS bundles. If you transpile JS, use babel-preset-env to avoid over-transpiling — this helps with both load-time and run-time performance.
Rendering performance is another important aspect.
I recommend the Web Fundamentals performance guide.