I also have this problem even with the 1.05 patch applied. People say install Directx 9 but like with yours it won't let me and it worked fine for over a year with Windows 8 and Directx 11 installed before this lag suddenlly appeared. Does anyone know how to fix this patch!?
I googled a bit on the topic and while I could't find a solution (except for installing DX9 which you couldn't for some reason, which by the way is really weird). However, I found some who claimed that this is due to a memory leak and they worked around it by aborting converstaion, saving and relaunching the game.
I know it's not a prefect, not even good, solution but it might be better than just living with it. Anyhow, I hope someone else can come up with something better.
