Not required, there is a console variable to alter this.

- composterUpdateInterval 

Default is 300 seconds, meaning 5 minutes, just alter that to the desired speed for the fertilizers to make compost, make sure to save the config and restart the server, it won't work without a restart.

I got that ConVar from the Composter class.

public override void ServerInit()
	{
		base.ServerInit();
		global::ItemContainer inventory = base.inventory;
		inventory.canAcceptItem = (Func<global::Item, int, bool>)Delegate.Combine(inventory.canAcceptItem, new Func<global::Item, int, bool>(this.InventoryItemFilter));
		base.InvokeRandomized(new Action(this.UpdateComposting), this.UpdateInterval, this.UpdateInterval, this.UpdateInterval * 0.1f);
	}

Where the UpdateInterval is:

protected float UpdateInterval
	{
		get
		{
			return ConVar.Server.composterUpdateInterval;
		}
	}

And the default value for this ConVar is:

public static float composterUpdateInterval = 300f;

So by altering this ConVar at the start of the server the speed of which the composter actually composts is determined by that variable.