Yes - it means that it has to do a double pass through the HTML so as to get the tfoot before it starts processing the tbody.
Even if it works it would slow the loading of the page while it gets the tfoot from where ever it is lower down in the HTML.
If browsers look for the tfoot further down the source then presumably always including a tfoot (even if empty) just after the thead would speed up the loading of the page as then it doesn’t need to read the table twice.