Hear me out. I am VERY new to all of this. I have just set up (BetterLoot,GatherManager,MagicCraft and No Decay). This is the first mod I have had difficulty getting it to apply changes in game. So I am willing to admit that more than likely this is just user error. Is there anyone here willing to help me troubleshoot this to better understand this issue?

Please append your config file

using System;
using System.Collections.Generic;
using Newtonsoft.Json;
using UnityEngine;
using Random = UnityEngine.Random;

namespace Oxide.Plugins
{
[Info("Quick Smelt", "Iv Misticos", "5.1.3")]
[Description("Increases the speed of the furnace smelting")]
class QuickSmelt : RustPlugin
{
#region Variables

private static QuickSmelt _instance;

private const string PermissionUse = "quicksmelt.use";

#endregion

#region Configuration

private static Configuration _config;

private class Configuration
{
[JsonProperty(PropertyName = "Use Permission")]
public bool UsePermission = true;

[JsonProperty(PropertyName = "Speed Multipliers", ObjectCreationHandling = ObjectCreationHandling.Replace)]
public Dictionary<string, float> SpeedMultipliers = new Dictionary<string, float>
{
{"global", 1.0f},
{"furnace", 100.0f}
};

[JsonProperty(PropertyName = "Fuel Usage Speed Multipliers",
ObjectCreationHandling = ObjectCreationHandling.Replace)]
public Dictionary<string, float> FuelSpeedMultipliers = new Dictionary<string, float>
{
{"global", 1.0f},
{"furnace.shortname", 1.0f}
};

[JsonProperty(PropertyName = "Fuel Usage Multipliers",
ObjectCreationHandling = ObjectCreationHandling.Replace)]
public Dictionary<string, int> FuelUsageMultipliers = new Dictionary<string, int>
{
{"global", 1},
{"furnace.shortname", 1}
};

Edit json in /config, not .cs in /plguins

By that you mean to do so in like notepad? not in the plugin folder itself right?