Hi Warsong,
Are you running on the latest patches for Sins of the Fallen (Alpha 0.2.2)?
There was an issue with the DEMO Plague race in the base release (Alpha 0.2) that chewed up CPU cycles. It would be more noticeable if you are having the AI play as Plague. This issue was fixed with Alpha 0.2.1 and several new features were added with Alpha 0.2.2 for Plague as well as some balancing issues with the Nephilim race.
If you are wanting to use the Plague race prior to Alpha 0.3 then I would suggest you keep up to date with the Alpha 0.2.x patch releases. The Plague race is only released as a playable demo and was not guaranteed to be without issues as indicated in the release notes.
Now if you are fully updated to Alpha 0.2.2 then I'll need more than what you have posted.
What is the map?
How many players are there?
What are the races for each player?
Please post a save game so I can download and test against my corresponding version of the mod.
Another note: I almost didn't look at this thread. Please post in the modd db downloads section as comments in the patch you are playing with or in the Sins of the Fallen thread on this site. It will guarantee a response from me in the future.