LESS, Media Queries, and Performance

Its almost impossible to give you a totally accurate answer.

When asking about performance the typical answer is to profile it and see what you get. Fix the slowest part first, then repeat.

You can profile CSS selectors in the Chrome Developer tools:

Ultimately I don't think the media queries will have anywhere near as much impact on performance compared to even a slightly complicated selector.

You can see more information about writing efficient CSS here:

https://developers.google.com/speed/docs/best-practices/rendering

Also with regards to file size and repeated CSS, gzip almost completely nullifies this as the gzip algorithm works best with repeated data.