You want to err on the side of convenience for the player in this sort of situation, which the developers obviously understand since they've placed a fair number of convenient portals around.
Issues with low movement speed are worsened when you have a party you can split up and move around areas, which is going to be happening constantly given the coop nature of this game. It's definitely slow in single player, how do you think players are going to feel when they're trying to catch up with their friends? "Oh man, this is so atmospheric waiting here as my friend slowly trundles up to this npc so I can initiate dialogue and both of us can interact in a conversation, he's half a map away and not sliding at all!" (as if that and other animation timings couldn't be easily tweaked haha) Increasing movement speed doesn't flat-out solve this, but it does provide mitigation.
You're given teleportation pyramids extremely early in the game, which allow you to regroup your party instantly. That's why they gave you two of them - one for each player character, with the ability to instantly send items (like the pyramids) to other character's inventories even when you're spread apart.. As far as addressing your above concern, they are far more effective than any minor speed boost will be.
along with a much smaller clickable area where the player walks instead of running as people have mentioned.
The walking radius is already small enough (see the image I provided earlier in this thread). While an "always run" option or toggle is fine for those that want it, making it even more difficult/tricky to walk isn't necessary.
