I communicate with my hosting again, and they reply below:
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You are hitting our abuse scripts quite often:
http://support.hostgator.com/articles/pre-sales-policies/rules-terms-of-service/processes-limit
Here are more database optimization tips:
http://maketecheasier.com/8-useful-tricks-to-clean-up-streamline-your-wordpress-database/2009/02/17
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Would you have suggestion on what I should do? I've set "Live Stream" option to 600 seconds already, so it'll refresh every 10 minutes instead of every 1 minute. Not sure what else I can do.
Many thanks.