After upgrading from moodle 1.8 to 1.9.3 views like
http://vc.uni-bamberg.de/moodle/mod/data/view.php?id=67166
became very slow (like 30 secs or longer). Load on the machine was 0.3 versus 0.8 when delivering such pages

ALTER TABLE `moodle`.`mdl_data_content` ADD INDEX `fieldid`(`fieldid`);
ALTER TABLE `moodle`.`mdl_data_content` ADD INDEX `recordid`(`recordid`);

seemed to fix it