Thanks again for all suggestions, the new version is now available for download http://wordpress.org/extend/plugins/related-posts-thumbnails with the following upgrades:
- Improved plugin performance with high number of posts. Database request was altered and plugin performance improved. So now on my local machine for 3k posts it executed for 0.017 sec in average, compared to before 0.2 sec.
- PHP configuration allow_url_fopen is taken into account. Some PHP configuration always returned default thumbnails, so now this option is checked.
- Added start date to exclude old posts
- Fixed display on pages with plugin setting for not single only.
- Separated categories – which are displayed amoung thumbnails and on which thumbnails are displayed.
- Random relation, for random posts display. It means you can set no relation at all to show any posts, not just with similar tags or categories.
- Plugin is now PHP 4.3 compatible according to WordPress requirements.
Plugin home page: http://wordpress.shaldybina.com/plugins/related-posts-thumbnails/