Most of NA and from what I've seen from EU, consists of heavy armor or at least, what used to be 40+ armor until the recent patch that nerfed it below 40(Heraldic Mail). Throw in gloves and looms and that means those people are 50+armor. Which isn't anything like what I'd call "medium" armor.
Furthermore, the soak factor was nerfed but the reduce was buffed. Have you not realized that you take twice as many hits compared to before the change? Plate allows you to live a hell of a lot longer, especially if you have the IF to go with it. I've had a fully loomed plate set and used it for more than just a couple gens, I know exactly what it does. You have to spend time adjusting to the speed difference but once you do, living through a minimum of twice as many swings as someone in a cavalry robe, is a pretty big advantage.
Speed may be great, but it doesn't allow you to get free hits in. Speed gets negated quite a bit by backpedaling and a simple mouse turn. You're not going to be able to get behind someone just because you're faster than them, if they're used to their own speed and are moderately skilled. Speed comes into play in two places, wpf and playing the range game, that is, dodging your opponents swing and following up with your own, before your opponent has recovered from his swing.
Due to WM and WPF mechanics, any gains from having light armor are generally extremely minimal. There are only a few cases where you can have a fast enough weapon to utilize the additional wpf that you have, without glancing constantly on your plated opponent due to low damage.
Dodging swings and then quickly counter attacking a slow player is only effective as long as your opponent has either ran out of patience, or didn't have any to begin with. Knowing your weapon's length and not getting drawn into the attack/block/attack/block rhythm is a valuable skill for plate wearers. Sometimes you have to know when to block two or three times in a row to catch up to your opponent, and simply knowing this gives you the tools you need to negate their speed advantage.
Simply put, speed does not necessarily mean free hits but plate does necessarily require you getting hit more times to be killed than if you had medium armor. What this means, is that the speed advantage comes about mostly due to a lack of skill from your opponent(the plated guy), meanwhile the plate/armor advantage applies to everyone in all situations, regardless of skill.
edit: consists of heavy armor, not medium, specifically 40-50 body armor base.
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