There is only once context of actual use where this is true: jousting. And this is because it was a sport that the competitors expected to walk away from.

Apart from that, what you said is a complete myth. Mobility what highly important on the battlefield, and armorers had to keep that in mind throughout the middle ages. There are many instances where troops acquire or are given pieces of armor that they later discard because it was too heavy or hindered them, and the weight you're imagining is not all that great. And this is not a problem unique to the middle ages - it still happens up to the modern day

As for full plate, it's not actually as heavy as you think it is because there are a lot of shaping techniques used to gain the maximum amount of strength for the least amount of weight. Generally Late Medieval full plate harness weighed 35-55lb Source

There are medieval reenactment groups such as the Society of Creative Anachronism (SCA) or Historical Medieval Battle (HMB) where people will regularly wear armor and fight. I myself do this, and my kit fluctuates between 40-55lb depending on what I'm fighting with that day. Adding that to my body weight results in a total of about 200lb, and dare you to claim that people at that weight can't move around.

What's even funnier is that groups like HMB tend to have armor that's significantly heavier than the medieval period because they need their armor to last a lot longer. In period, men at arms only needed their armor to last a few battles, while reenactors look to use their armor on a weekly-to-monthly basis for years