Demon Souls on 0.0.10.x

Doesn't seem related, I can't get passed loading
Running the latest fps patch but removing this doesn't affect. Tried all sorts of config and always get the same result. Also cleared cache

So weird, maybe someone else can help us. Try with a clean copy of the emulator, download the latest version and the firmware and try the game.

In my case I don't understand why the game fall to 10fps or less, but in background goes very smooth.

You didn't say what you're running if it's an Nvidia card in the Nvidia control panel if you go to Manage 3D Settings -> programs tab, find or add Rpcs3, scroll down to Power management and channel it from Optimal power to Prefer Maximum performance.

I'm running AMD but had the same issue on Nvidia card I replaced a few weeks back