On telephoto, setting, the mirror lockup special function gives you an additional way to reduce camera shake, there will be no mirror slap to cause motion-induced blur.
It hard to imagine the setting where 1/20 exposure would be required for such a shot in sunlight, even at golden hour. Are you stopping way down?
ISO was 125, aperture was at f/8, normally the sweet spot for sharpness on this lens. I normally shoot as far to the right as I can go on the histogram, just before I start blowing highlights. I'm going to do a few tests tomorrow to see what's going on. I'm thinking one of the legs was just in some soft ground (it was fairly wet up there). I'm used to much sharper images than this.