To test the browser rendering time of media queries, 2 stylesheets file were created with a php script render-media-query.php.
The files styles-combined.css and styles-media-queries.css has the same amount of styles in them.
The only difference is that styles-media-queries.css has only one grouped media query, and styles-combined.css has the same media query around each class.
Of course the size of the stylesheets files differ: 9.6M vs 12.5MB. styles-media-queries.css is 30% larger than styles-combined.css.
However, as you can see from the Developer Tools example, the parsing time of style sheets: 265.5ms vs 416.9ms. It does take 57% more time to parse the unoptimized version of the stylesheets file.
This is in no way a real world example, I'm sure, and do hope, that no site will ever have 99999 style classes.