Go to the layout directory in app/design/frontend/default/* (your current theme), edit page.xml (if not there, copy from base directory) and modify the following:

Change:

<block type="page/html_breadcrumbs" name="breadcrumbs" as="breadcrumbs"/>

To:

<!-- <block type="page/html_breadcrumbs" name="breadcrumbs" as="breadcrumbs"/> -->

Magento can be a beast. Every time a potential customer visits the website, several SQL queries run to log his details in the DB.
Not only that this is unnecessary (most of the details, except for the IP, are tracked by Google Analytics anyway), this could be a serious performance issue.

How to deactivate?

Copy /app/code/core/Mage/Log/Model/Visitor.php

To: /app/code/local/Mage/Log/Model/Visitor.php

Change the 4th line:

protected $_skipRequestLogging = false;

To:

protected $_skipRequestLogging = true;

Typically, when a Magento store reaches 20,000 orders and more than 5 people load the sales-orders grid concurrently, our online store becomes extremely slow.

How do we solve it? simple. limiting the amount of orders presented in the orders grid for the last 24 hours, unless requested otherwise.

Copy the app/code/core/Mage/Adminhtml/Block/Sales/Order/Grid.php file to:

app/code/local/Mage/Adminhtml/Block/Sales/Order/Grid.php

Edit the following function, copy-paste from here:

protected function _prepareCollection()    {

$collection = Mage::getResourceModel($this->_getCollectionClass());

######################## FILTER BY LAST DAY ######################
$now = Mage::getModel('core/date')->timestamp(time());
$filter   = $this->getParam($this->getVarNameFilter(), null); //important - check for other requested grid-filters before filtering today's orders

$dateStart = date('Y-m-d' . ' 00:00:00', $now);
$dateEnd = date('Y-m-d' . ' 23:59:59', $now);
$postData = Mage::app()->getRequest()->getPost();
if (empty($filter)) {
$collection->addFieldToFilter('`main_table`.created_at', array('from' => $dateStart, 'to' => $dateEnd));
}
##################################################################

$collection->getSelect()->group('entity_id');
$this->setCollection($collection);

return $this;

}