This tutorial will show you how to merge and minify CSS files in Magento 2 store. Please follow below steps to merge and minify CSS files in Magento 2.
1) Log in to Magento 2 using your admin account.
2) From left sidebar, click Stores, and then Settings -> Configuration.
3) In this page, expand Advanced section in the panel on the left and choose Developer.
4) Now select CSS Settings tab.
5) Set Merge CSS Files to Yes.
6) Set Minify CSS Files to Yes.
5) Click on “Save Config”.
6) Clear the cache.
Thats it. Enjoy Magento 2 speed.