Is there a way to slow down the default running speed by say 10% for both player and all NPCs?

[peppergomez]

And boost walking speed by say 10%, ditto? Walking feels a bit too slow and default running (not sprinting) feels a bit too fast.

 

If this is possible, will it be safe to do so without resulting in any game instability? Thanks.

[EssArrBee]

You would have to change a game setting. 

Here's a mod that does some adjustments: https://www.nexusmods.com/skyrim/mods/1931/?

Character Sneak Walk +15%Character Sneak Run -25%Character Walk +15%Character Run -20%Character Sprint -20%Horse Walk +5%Horse Run 0%Horse Sprint +5%

[Kelmych]

The mods Realistic Running Speed and Realistic Humanoid Movement Speed change various speeds. If you load them and look at the Skyrim parameters they change  (which are GMSTs) in TES5Edit you will see how to change a wide range of speed parameters. It is quite straightforward, at least with TES5Edit. Of course, if you like the default parameters one of these mods uses then you don't need to edit anything. The kinds of changes you want to make should be safe.

[peppergomez]

Thanks. It's important to me that these changes also apply to NPCs....I don't want to now be running slower than they are (realizing that some NPCs will run either slower or faster than my character, by design, of course)