Open includeJS for editing,
Find this code
{{foreach array_unique( array_filter( \IPS\Output::i()->jsFiles ), SORT_STRING ) as $js}}
{{$js = \IPS\Http\Url::external( $js );}}
<script src='{{if $js->data['host'] == parse_url( \IPS\Settings::i()->base_url, PHP_URL_HOST )}}{expression="$js->setQueryString( 'v', \IPS\Output\Javascript::javascriptCacheBustKey() )"}{{else}}{expression="$js"}{{endif}}' data-ips></script>
{{endforeach}}
and replace with
{{foreach array_unique( array_filter( \IPS\Output::i()->jsFiles ), SORT_STRING ) as $js}}
{{$js = \IPS\Http\Url::external( $js );}}
<script src='{{if $js->data['host'] == parse_url( \IPS\Settings::i()->base_url, PHP_URL_HOST )}}{expression="$js->setQueryString( 'v', \IPS\Output\Javascript::javascriptCacheBustKey() )"}{{else}}{expression="$js"}{{endif}}' data-ips></script>
<link rel="prefetch" href='{{if $js->data['host'] == parse_url( \IPS\Settings::i()->base_url, PHP_URL_HOST )}}{expression="$js->setQueryString( 'v', \IPS\CACHEBUST_KEY )"}{{else}}{expression="$js"}{{endif}}' as='script'>
{{endforeach}}
Note that if you use 3rd party plugins, or use ads, you will need to make another template in which to include all external java scripts, which also create a TBT increase.