Most armor rigs (excluding nano pumps and accelerators) apply a speed reduction as the rig penalty. Armor ships are normally already slower than shield ships from a baseline (unless comparing fastest armor to slowest shield), then you add in plates, trimarks or resistance rigs and it penalizes armor speed even more by just using the preferred tanking method of the ship.
In most cases, a single plate does not massively affect speed. Even the most hated nanoboi’s will put 800 plates on kitey ships and lose only a negligible amount of speed. The main speed debuff for armor is in the rigs.
Why does this matter?
To create better diversity between armor/shield engagements where 1 isn’t more preferred due to raw mobility and the other can be kited indefinitely just for using its main tank. I think in the current meta, involving HACs, an easy example to make is between the zealot and eagle. The eagle is arguably one of the slowest shield cruisers in the game, yet once you put plate/rigs on the zealot (or sac), the eagle is faster than it. So the eagle can project farther and can just kite away from the zealot, meaning the zealot is really not viable in this fight. I know the zealot has more issues than this, but its just an example.
Eagle speed w/ MWD = 1582
Zealot speed w/ MWD = 1676
Zealot speed w/ MWD (1600 plate/trimarks) = 1356
Zealot speed w/ MWD (1600 plate only) = 1493
By changing the rigs, the Eagle is still faster, but instead of a 200m/s+ difference, its less than 100m/s. This could be tweaked by buffing zealot speed or nerfing eagle speed pretty easily to make them more competitive.
This same comparison occurs to things like harb vs ferox
Ferox speed w/ MWD = 1073
Harbinger speed w/ MWD = 1229
Harb speed w/ MWD (plate/trimaks) = 1013 m/s (would actually be even slower with 3rd trimark, but heavy beam fit only uses 2 trimarks+1 ACR).
Harb speed w/ MWD (plate only) = 1114 m/s
With plate speed penalty only, the harb is now faster than the Ferox and can actually get into range with heavy beams and not get slowly kited to death.
These similarities occur through pretty much the entire EVE ship line. There are exceptions of course, but they are limited.
While we’re here, since armor would lose the speed penalty, apply the speed penalty to hull tanking bulkheads (low slot mods, not the rigs). This way the max bulkhead hull tank ships become slow bricks to match their equally absurd and cheap EHP values.
The agility penalty would still provide a reasonable drawback to fitting trimarks/resist rigs, but doesn’t nerf an armor ship’s ability to catch up to a shield ship completely. It would mean the armor ship can’t react as quickly to sudden changes (warp-ins or bombs), but still be competitive in speed. It also means if the shield ship is more active and purposely burns perpendicular to the armor ship, it can still shake off an armor ship.
This changes the dynamic to shield vs armor and can affect how they engage each other. Where armor you want to try to move around on them to keep them working with their low agility, but if they need to reposition or burn out, they can still achieve reasonable speed to do so.