The thing about a mouse interface is that it's a lowest-common-denominator approach for a lot of things that works pretty ok. Certainly better for that than the text-only interfaces it's largely replaces. This is also why the web is so married to it, 'cause you can navigate around a bunch of semi-consistent information.

The average confused user will probably do better randomly mousing around than trying to juggle keys, or even worse "visually" navigate using arrow keys or something.

But if the interface is powerful and the user is skilled (both significant "ifs") then important operation can be done before your hand would reach the mouse, let alone do anything with it.

I suspect the most efficient is a mix of both, mice are pretty good at selecting from a long list of options, for example.

A lot of IDEs these days contain pretty underpowered editors and slow you down by relying on the mouse too much. If that's all you've tried, you may not have any idea why keyboard interfaces can be a lot more efficient for some tasks....