My suggestion is to change the single game and optimization pages to allow to hide/show games by clicking on group headers, like “Tutorial”, “Easy”, etc.
This will allow to easily find challenges based on the level, and to quickly “filter” challenges.
We asked for it since 1986 ! 
I heard it’s already scheduled but not in the priority queue.
Or square tiles like this:
With hover text and minimal separation lines.
Nice Idea 
A little more spacing perhaps for other details
Yes, it’s just an idea of a compact layout.
The hover text should be larger than the thumbnail and a coloured vertical line on the left should show the hardness.
The hardest part was to download the thumbnails. 
I’m not a huge fan of the suggested tiled layout, because I cannot imagine how it would let you know in a glance what games you have already started or finished.
And in the same way, the new “multiplayer games” pane is great, no longer hidden in the “contests” menu. It would be great if it could use the same layout as the two other panes though, with just a thumbnail and some participation information (for example the ranking and score, like for optimization games). It would make it a bit more compact and more convenient. (But I assume that there already are plans to tweak this pane, at the moment they merely moved the old page around on the site.)
It will be done the next release.