question about cpu usage

I'm not a hardware guy or sys admin person so forgive me if this is a stupid 
question. Leaving out all other variables(such as IO), should I expect 
performance to be the same in a databse if the server it is riding at is at 90% 
cpu usages as opposed to 10%? since there would still be spare cycles? Or is 
there a declining returns as you get closing to the maximum available cpu 

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