I confess to myself - I have forgotten this topic... But there is one accidental, or not quite solution that I have found.
And here I am sharing it with you.
Open the Include CSS template
Find it
{{foreach array_unique( \IPS\Output::i()->cssFiles, SORT_STRING ) as $file}}
<link rel='stylesheet' href='{expression="\IPS\Http\Url::external( $file )->setQueryString( 'v', \IPS\Theme::i()->cssCacheBustKey() )"}' media='all'>
{{endforeach}}
{{if \IPS\Dispatcher::i()->controllerLocation == 'front'}}
{{$customCss = \IPS\Theme::i()->css( 'custom.css', 'core', 'front' );}}
{{foreach $customCss as $css}}
<link rel='stylesheet' href='{expression="\IPS\Http\Url::external( $css )->setQueryString( 'v', \IPS\Theme::i()->cssCacheBustKey() )"}' media='all'>
{{endforeach}}
{{endif}}
And replace code with
{{foreach array_unique( \IPS\Output::i()->cssFiles, SORT_STRING ) as $file}}
<link rel='preload stylesheet' href='{expression="\IPS\Http\Url::external( $file )->setQueryString( 'v', \IPS\Theme::i()->cssCacheBustKey() )"}' as="style" media='all'>
{{endforeach}}
{{if \IPS\Dispatcher::i()->controllerLocation == 'front'}}
{{$customCss = \IPS\Theme::i()->css( 'custom.css', 'core', 'front' );}}
{{foreach $customCss as $css}}
<link rel='preload stylesheet' href='{expression="\IPS\Http\Url::external( $css )->setQueryString( 'v', \IPS\Theme::i()->cssCacheBustKey() )"}' as="style" media='all'>
{{endforeach}}
{{endif}}
You will eliminate render blоcking CSS without much effort...
Of course, there are other little (hard to do) tricks for even better performance, but the above will also make you smile…