https://developers.google.com/speed/pagespeed/insights

Is very useful in testing your web site speed. It will give you all the information about why your website is slow.

The most easy way to make your site faster is compress your images.

Optimize images

Using small size images can greatly reduce your download time.

Gzip compress

Enable Gzip compress is the first task you need to consider, https://developers.google.com/speed/docs/insights/EnableCompression.

Using min.js instead of the full version js package

Usually mini version js already covered your most usage, using full version will generate more server load and takes more time for downloading, using CDN version is also a good idea to reduce the traffic.