Our system admin found that if you add another index (using "originalquestion" field) to the mdl_question_states table, it would improve the quiz quite a lot if you are running a large moodle site. I would recommend this as default to create index with this field. Currently the default indexes that will be created by moodle installation script for this table are id (PRIMARY), attempt (mdl_quesstat_att_ix), and question (mdl_quesstat_que_ix). Thanks!