I think there may be some generalisations here about "all" or "most" mixtes. Fact is, there are two very distinct geometry types out there - one being more leisurely and the other definitely more aggressive. Some of that is down to era of manufacturer. For instance, my 1970s Puch mixte is definitely quick (while still being comfortable) - suitable for long road rides - but doesn't feel quite "right" when set up with North Road bars (and a more upright position). Whereas today's Linus and Public iterations (as two examples that come readily to mind) are much more upright and 'about town' in character, as a result of their basic geometry, supplemented/enhanced by the components they're built up with.
You may find reading this helpful.
And the same blogger's write up of her Soma BV test ride here.
Rebecca
Riley - custom 2014 Enigma Etape
Bridget - 2010 Surly Cross Check
Lorelei - 1979 Puch Princess mixte
Astrid - 2014 Viking Bromley singlespeed mixte
Lucy - bespoke 2012 Brompton S1L
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