McAfee Viruscan question

[DomainEmpire.com]

I've just installed McAfee Viruscan 7.1, this's my first experience with this antivirus.

Well, I enabled it to working fulltime "Enable Viruscan" but I see my Pc is much slower now including all softwares that I use
Is this normal ? Should I fix this problem by enabling/disabling some particular options ?

Thanks for helping.