Improving the performance can be made by using analysis tools and using the information to improve the performance where the most gains can be obtained.
- Browser net analysis
- PageSpeed Insights
Page sight insights is provided by Google and will analyse your site and provide a lot of background information on how it may be improved.
The analysis provides information on how many bytes may be save using minimisation and it seems best to go for the biggest problem first.
The site will also carry out optimisation and provide a downloadable file with the optimised resources.