I’m sure you’re right about weight and speed. In fact, I’m so sure you’re correct, that I’m pretty sure knights in full-plate armour were basically tanks, and it was most effective on horseback (where the horse was your feet).
In other words, your’e so correct about the movement and weight, that their movement was so restricted, they basically just relied on the armour the save them from being hit, rather than avoiding the strikes.
I’m also there’s lots of different variables, and it would depend on the helmet design. There were thousands of different helmet designs throughout history. And although I never trained with a medieval helmet on, I do remember a university history professor telling us (many years ago now) that the limited peripheral vision with various helmet designs throughout history was in fact a very real issue.
It’s a trade off between protection and visibility. You can’t expect to see much at all wearing one of these! I’m more than happy for your vision and you ability to move to be both hugely nerfed when wearing this type of armour in the game.