I just experimented with Capture One 11 (11.0.1) and am very pleased to say that the preview quality issue has been solved for me.
My previous observations and comments were regarding C1 10 (and C1 9). However, it appears Phase One has changed preview generation in C1 11. EDIT: That's what I thought initially as I seemed to have found a preview image size setting that ticked all boxes: (Relatively) sharp previews and no issues during image adjustments, even at high magnifications. It turns out, though, that often image adjustments become very bothersome if the preview image size is set too low (see my
later post).
It is still possible to recreate the same issue for vertically oriented images by toggling the image information bar on and off. Say, the image looks fine without the image information being shown, as soon as you display the image information, it will look slightly blurry. However, if you zoom in (say to 100%) and then zoom out again, the image will look crisp. If you now disable the image information bar again, the image will look slightly blurry. This is not a problem at all, though, as zooming in and out will fix that (as before).
So apparently, now C1 recognises your "Fit" size (the latter depending on whether additional elements, such as for the image information steal vertical space) and creates an optimal preview for it. Perhaps this is just a consequence of me having found a sweet spot for the image preview size, but whatever causes the new behaviour, I'm taking it!
For those who were suffering from bad preview quality in C1 10, I'd strongly recommend trying C1 11!
EDIT: Depending on whether you'll also get the bothersome experience during image adjustments at high magnifications (at least sometimes), you may not be satisfied just yet. I hope Phase One keeps working on this issue.