In other words you should minimize number of bytes. The fewer bytes we need to download, the faster browser can get data and start processing to paint the page. You should keep HTML size as low as possible (>14KB would be ideally - read more why it's so magical value here)
Website won't be painted when CSS files still fetch. There are two ways to get rid of this issue:
I believe your website is responsive in terms of RWD pattern (Well, it's 2019 and you have to upgrade if it's not). In this case you use breakpoints.
<link href="main-sm.css" rel="stylesheet" media="(max-width: 639px)"> <link href="main-lg.css" rel="stylesheet" media="(min-width: 640px)">
Results in implementation of faster WordPress tips can be achieved also on server side. "The primary goals for HTTP/2 are to reduce latency by enabling full request and response multiplexing, minimize protocol overhead via efficient compression of HTTP header fields, and add support for request prioritization and server push." says Google experts. In other words it's faster than HTTP/1.1 because it allows for parallel assets loading. It means that there's no need to use assets concatenation. Below you can see why having HTTP/2 on your server is so important.
One of the simplest methods to speed up WordPress. PHP 7.x is faster than 5.x. No additional comment, see tweet below:
Are you still running your #WordPress website on PHP5.6? It's no longer officially supported and you will see huge performance and security benefits with PHP7+. All good hosting companies should provide the option to upgrade - if not, look for a new host! pic.twitter.com/yHbBKXQGgy— WPExeter (@WPExeter) 12 December 2018
I recommend Kinsta servers.
In other words it's TTFB (Time To First Byte). Lower is better. It's great to serve content < 300ms. When timing is higher than 1000ms you should definitely consider optimization.
It's a time of idle when content is processing by PHP parser and delivered to browser. In eyes of user it's moment when blank screen is visible. Feel free to verify your TTFB timings across the world here. Common solution to reduce TTFB is cache installation (WP Rocket or similar). You probably discovered that caching can only mask this issue. What if user access non-cached section of your website? WordPress speed is crucial in such a case. There are typical solutions for your problem but I believe you're here because your are looking for help with it.