Ask your wife if it's a problem. Dollars to donuts she'll say it's not even if it is but wouldn't it be cool if she was honest enough to tell you it was? Most importantly you need to communicate about what's brothering you. For all you know she thinks your short fuse is just fine with you and she's not bringing it up because she doesn't want to hurt your feelings.
12 minutes isn't bad if you can reliably manage both orgasms. Believe it or not the vagina isn't designed to be able to sustain sex for more than 15 minutes. Most women should be able to attain orgasm within a handful of minutes or penetrative sex given proper foreplay. If your wife needs more stimulation you shouldn't feel that your penis is the only thing that can please her.
Talk it out with her focus on a solution rather than a problem.