I suspect that something, or maybe many things are not quite right. It could be an accumulation of everything already mentioned, and more.
Do you have a stock steering wheel, or a smaller one? A smaller diameter wheel will decrease your leverage, and more muscle will be needed.
The tires are new, but are they the standard diameter and width, or are they smaller and wider? A tall narrow tire will turn easier than a low profile fat one.
What about the bushing on the steering shaft? If that is badly worn, it might be increasing drag (resistance). Just a theory, but when you are applying all that extra might, you are actually pushing, or pulling the steering shaft against the bearing or (bushing).
And, as already mentioned above, lubrication is extremely important.
John
