OK - I understand why running on a shared host would produce those statistics. Maybe you could approach the Siteground helpdesk with these figures so that it can help them when reviewing the performance of that server? The mdl-db-opt script can be run in a production environment. It performs the mysql check, optimize and repair operations on each of the moodle tables.
It may, but probably may not, help - as it looks like the problems are related to server overloading and a lack of indexes in the database. http://moodle.org/mod/forum/discuss.php?d=58208 contains a summary at the end of missing indexes in some moodle tables - but this is for moodle 1.7, so (assuming moodle 1.8 has all those indexes) it again may not improve things.
Good luck!
Hardware and performance
please Help with MySQL optimization
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