Actually, if on "optimal" your machine is using a lower refresh rate and you then tried it at 75, your machine may be demanding more video information from the host and it can't keep up. I had a similar problem a few years ago playing Fighters Anthology on line. using higher resolution and detail would lag the game bad. but setting lower res and detail everything was smooth.

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Who the hell is General Protection and why is he messing with my comuter?