There are couple of things you can do,
Enable gzip compression: It reduces the size of files sent from server to increase the speed. You have to enable it in
.htaccessfile, for more information visit this page.
As @2pha said
Advanced CSS/JS Aggregationis very effective effort.
Install optimize_db module: it handles the issue with fast-growing table cache_form.
Anyway, gtmetrix suggests many techniques to improve your website performance.
What have you tried already?
I use the Advanced CSS/JS Aggregation module which combines js and css file together making for fewer http requests. It also has some other features including moving js and css to the footer.