Re-created slow-mo mod for .Net.
-
Left it uncompiled for easy reading/editing.

You will need 
https://www.nexusmods.com/reddeadredemption2/mods/70
and its requirements

And then drop this file into the scripts folder.

I also added a horse speed increaser.

open the file in any text editor and change the info in the top to your tastes.

/// Change these values only
        int MouseClickDelay = 50;   //Delay between recognised mouse presses so holding a mouse button doesn't toggle too quickly
        int HorseSpeed = 20;        //Too fast will just slam you into small rises in the ground very hard
        Keys HorseKey = Keys.H;     //This is the key to increase horse speed (Only works while on a horse, turns off if you dismount
        MouseButtons mbHorse = MouseButtons.None;
        //MouseButtons None, Left, Right, Middle, XButton1, XButton2
        float fSlowSpeed = 0.2f;    //above 1 does nothing
        Keys Slowkey = Keys.Y;      //This is the key to activate slowmode
        MouseButtons mbSlow = MouseButtons.XButton2;
        /*
        Extra silliness
        F12         -   toggles keychecking so you can get what keys are available to bind
        N           -   Sets time to midday
        Numlock     -   toggles targetting for use with below
                    -   Once on, right click and hold or looking at with binocs or pointing a gun at will select a PED or Vehicle
                    -   target will be remembered
            Once you have a target
        Numpad /    -   Freezes the target (use on the horse or vehicle if ped is mounted)
        Numpad *    -   Unfreeses the target
        Numpad 4    -   Booms the target
        Numpad 5    -   Zaps the target (sometimes takes a while to arrive)
        Numpad 6    -   Kills the target (will not damage pelts)
        Numpad -    -   Changes the explosion type for Booming. Decrement
        Numpad +    -   Changes the explosion type for Booming. Increment (max seems to be 34/35)
        */