This is the customer that is used when search engines access the site So if it is searching and accessing products and the estimate shipping option is enabled then maybe it will be trying to caclulate tax for the shipping ?? One option as a test may be to customise the plugin and if the customer is the above then dont try to get the tax ?
Yes, the slow site speed is likely a common issue with the Avalara plugin, given the reported problem. To fix it, check for plugin updates and see if others have experienced the same issue. Also, optimize your website's performance for better results.