Jan, also if you look in the same database (File>New IQ Base with Sample Data), one of the grids shows a time tracking example. There is a column for start and end time, if you click on them they select the current time. Once both are populated, duration (column) is calculated automatically. You can also manually enter a duration.
As far as "fast", personally if I want to enter a fast time entry that I don't feel it necessary to track in a dedicated grid, I go to the Done grid and enter it there. You could also create a grid called "time tracking" and enter it there.
The time for child items will also roll up to the parent item.
Finally, you can use pivot tables to perform more advanced calculations such as all time entered for the week or month or whatever. But I think that goes a little beyond your question.
p.s. You will just want to add the following columns to whatever grid you want to track time in:
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