'Dead on' emphasises that the performance will start at exactly that time. The performance will start dead on six ie. It won't before six and it won't start after, but exactly on six o'clock.

"The performance will start dead-on at six." would be the correct way to write it. It means that the performance will start exactly at six.

I think it means that the perfomance will start strictly on six without delay.

Dead on - derives from the accuracy of a shot. 'I hit the target dead on' meaning i struck the target precisely where I aimed it, or in it's exact center. When it is used in reference to time, it refers to the position of the hands of the clock, 'The bell rings dead on 12 o'clock'. Because the metaphor is pretty basic, it became synonymous with 'Precisely' six o'clock.

there is no need to know the context to understand meaning of your sentence. Good sentence