When i use the mouse, clicking in the column, the response is immediate. No lag at all. But that is apart from moving the focus between images. I just made a test in a smaller test folder, with 10 images, and the issue didn't occur. Then, in other folders, the issue is back again, but not constantly. But i have to note, that i am changing the focus by using the standard arrow keys, unlike you, clicking on them. Which also works fine here, when i hover over the files, it would display the correct image.
And i must have changed the F7 key a long time ago to F3, i don't even remember the reason. My F7 actually creates new folders. 
I suppose, i had to juggle around with my hotkeys for some reason back then, but i'm so used to it, i wasn't even aware, that they were changed. But at least F2 is still for renaming.
So, i have no idea why i see this effect here sometimes and maybe in more crowded folders. I would use the mouse to rate otherwise, but i have to aim at the left side, while evaluating the preview pane on the right, so the keyboard thing is more convenient. But, while writing this post i have found a way to avoid that lag, by having changed my buttons commands back to the way it was some time ago, where i had an additional select next command, like so:
SETATTR META rating:3
select next
Which looks like the issue happens with the up and down arrows sometimes. Although the keys are not broken, i'm sure, as this is an almost brand new Cherry keyboard. Thanks!