How to increase or decrease a players speed

So, I'm using DTM toolkit in my mod, and it works great for multiplying damage and defense and stuff like that, but it doesn't seem to have a speed editor... Meaning that I want to edit the player's speed on certain conditions. Say the player puts on a pair of boots, I want it to basically make the player slower without giving an effect. Then, when the player increases their speed stat to enough, the speed dampening will eventually go away. I hope I explained clearly. Is there a way to do this? Thanks!