According to discussion in https://codereview.chromium.org/1706193002/ it looks like some download item fields are never changed after download item is created. This means that those fields are never present in download extension api onChanged event delta and can be removed from there.

Things that are constant after creation:
Download URL chain -> url and finalUrl fields in delta.
Mime type -> mime field in delta.
Start time -> startTime in delta.