Hi, thanks for sharing this. I can see the annoying limitation here.
The Processing Time column is actually a massive over-simplification of several important data elements. It simultaneously represents the 'expected processing time' and the 'actual processing time'. On fixtures, it will report only the processing time related to the fixture as a whole (i.e. TestFixtureSetup, TestFixtureTearDown), while as you've mentioned, the processing time of child tests are reported separately.
Reporting the fixture setup times as a whole becomes problematic if the child test execution times are included. The only real answer is to split this column up, so we have a 'Total Processing Time', 'Expected Processing Time', and 'Last Processing Time'. Sooner or later this will need to happen.
If you're running V2, something that may help you here is to change the grouping setting on the Tests Window to 'Test only'. This will allow you to sort all tests in order of their processing time, making it easier to find the ones that take a long time to run. You could combine this with the list of fixtures sorted by processing time to help you find fixtures with a long setup/teardown time.