Own theme very slow

We use Vaadin 7.7.11 portlets on Liferay 7.1 (portlets migrated from Liferay 6). We don’t want to use any of the vaadin themes, we have all necesarry styles in our own theme (not a vaadin theme). So we created a folder mytheme in …/webapps/ROOT/html/VAADIN/themes, put empty styles.css, styles.scss, legacy.css and mytheme.css there. In the portlet class we annotated
This works, the portlets are displayed but the loading time is very long (6-9 sec).
If we annotate for example
the portlets are displayed within 2 sec but the layout is broken, because the styles of runo theme destroy our own styles.
Is there a possibility, to not use any of the vaadin themes?