Lately have been getting a black screen before every loading screen, not using steam, am on windows pc, task manager shows there's isn't anything eating significant resources other than Spiral Knights (power usage seems to spike to very high after logging into account, and other internet connection dependent applications slow down significantly).
The black screen is sometimes eternal and sometimes absurdly long. When it is long, loading occurs slowly.
By process of elimination in game options, compatibility mode and windowed mode unfortunately aren't working anymore, but aside from that the game plays normally again :)
EDIT: Compatibility and windowed are working, but to make the game load levels I need to leave vertical sync enabled...? OK! Works for me!