According to the Red Box rules, an unarmored character moves at a rate of 40’/round, one in leather armor moves at 30’/round, and one in metal armor moves at 20’/round—or a mere 10’/round if also carrying treasure. But how fast is that?
A round is ten seconds, so an unencumbered man walks at a rate of four feet per second. Running triples one’s movement rate, albeit at the cost of temporary fatigue (-2 to attack and damage rolls and to AC), so an unencumbered man runs at a rate of twelve feet per second. These are reasonably accurate numbers, all things considered. (Sure, some people will walk or run faster than others, and there’s jogging and sprinting and so forth, but this is Basic D&D; we’re not going to fret the details.
How about a character in metal armor? Halved movement speed is pretty extreme. Personally, I don’t have much—or, in fact, any—experience with moving around in plate mail. A bit of casual internet research (and we know how accurate that is!) suggests that heavy armor doesn’t slow one down significantly; the main effects are an increased demand on the wearer’s stamina from hauling around the excess weight.
But let’s face it: sometimes we deliberately ignore realism to keep gameplay simple and to produce interesting tactical or strategic choices. The movement rules may not reflect reality terribly well, but they’re simple and they work. Want to move fast? Wear light armor or none at all, or run in armor and accept the resulting penalties. Want to hang tough on the front line? Wear heavy armor. Can you make it out of the dungeon laden with treasure? Let’s find out!
And here we encounter the one fly in the ointment: chain mail. By the book, there’s little reason for player characters to ever choose to wear chain mail. It costs only 20gp less than plate (a trivial savings) and weighs only 100 coins less than plate (allowing one to bring out a little more treasure), while providing significantly less protection in battle.
My solution has been to house-rule the movement table. In my game, characters in plate move 20’/round, characters in chain move 30’/round and characters in leather or no armor move 40’/round. Accurate? Unlikely. Playable? Definitely! Chainmail suddenly becomes a viable choice, as the character wearing it gives up protection to gain significantly increased mobility. (My players had an example of this last session, where the plate-wearers slogged slowly through a storm of arrow-fire to reach their opponents.)
Other solutions are certainly viable. One could use AD&D-style tables indicating which weapons are best against which types of armor, or one could modify the exhaustion-from-running rules to impose greater penalties on plate-wearers. I’m curious to see what approaches individual referees have taken in their own campaigns!