For appropriate assignments (such as where students are just typing in answers and no special formatting is required) I use the Online Text Assignment and in the options I select the "Comment inline". When I'm marking I add my comments into the text box area using red coloured font. It's fast, easy AND efficient.
If you have to do heavy editing it might not work as well.
The downside is that in the entire submission AND comments will appear in their "report" area.
Gradebook
Rapid Editing of Submitted work
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