Here's a bookmarklet that should allow you can see how long the subtotals will take per subtotal.
This bookmarklet opens in a modal-type window directly over top of the To-Do List page you're currently viewing.
It will work in all the major browsers e.g Firefox, Google Chrome, Internet Explorer, Safari and so on.
then after any sort in Toodledo you can see how long the subtotals will take per subtotal.
For instance, if you sort by Context or Tag then clicking on your bookmarklet will tell you how long you've estimated
it will take -- in total -- for each Context or Tag.