One thing I must ask, when you say "the bolt is more difficult to open", do you mean unlocking the bolt is more difficult or drawing it to the rear is more difficult?

If unlocking is more difficult, then there is nothing wrong with your gun, most modern arms recock the firing pin as you unlock the bolt (rotate the bolt handle), but you shouldn't notice any hanging or increased resistance when drawing the bolt to the rear.

If it is hanging up as you draw it to the rear, then there's something strange happening, because really NOTHING is happening to the firing mechanism as you're drawing the bolt rearward--certain guns (most mausers) still cock the firing pin as you push the bolt forward instead of as you unlock the bolt handle, but there aren't any that I'm aware of that do anything while you're drawing it rearward other than eject the casing, and that resistance would be constant regardless of whether you've fired it or not.