C1 V4 also rendered full resolution up from 25%... below it has always been a bit washy.
In V5 you can set the preview size in the preferences:
if you have a large monitor and don't use the proof margin you have to set a higher value... according to the pixel size "to fit" utilizes.
The smaller the preview cache size, the faster, the bigger, the slower. The balance quality vs. speed is up to you.
I messed with this slider and there is no change in preview sharpness or size that I can tell.
as all the tools: copy & apply the adjustment to the selected images.
Except...it doesn't work And these are simple dust spots - on a plain white background, in the same place each time. When working with an individual image, clicking on the dust spot puts the circle around it, and does nothing. But, if I leave the image, and come back to it, sometimes the spot will be gone. And sometimes it won't. Despite always being on a white background.