We've just released a patch that will now cause the graphics quality level to default to 'Medium' instead of 'Ultra'. If you've loaded Outbreak: Lost Hope prior to this patch, make sure to go to the Options menu and confirm your graphics quality level prior to playing.
Although the Ultra graphics quality level will look excellent and provide additional real-time shadows, it is extremely demanding and should only be used by folks with high end PCs. Medium quality will still look great so it's the recommended default.
You should also confirm your settings for anti-aliasing as that can impact performance if it's set too high on lower end or medium tier machines.