(Updated) precache the next entry in your playlist if it is a network source by downloading it to a temp file ahead of time. Make sure to change Line 1 to a directory where you want the files to download to. This update makes it much more usable by downloading all streams in parallel so you can watch it while it downloads. Only downside is it do…

ytdl-preload.lua

local cachePath = "R:/ytdl"
local nextIndex
local observeInitial
mp.enable_messages("info")

local function listener(event)
	if event.prefix == mp.get_script_name() then
		local destination = string.match(event.text, "%[download%] Destination: (.+).mkv") or 
		string.match(event.text, "%[download%] (.+).mkv has already been downloaded")
		if destination and string.find(destination,":\\") then
			mp.commandv("loadfile", destination..".mkv", "append", "audio-file="..destination..".webm")--,sub-file="..destination..".en.vtt") --in case they are not set up to autoload
			mp.commandv("playlist_move", mp.get_property("playlist-count") - 1 , nextIndex)
			mp.commandv("playlist_remove", nextIndex + 1)
			mp.unregister_event(listener)
		end
	end	
end

local function DL()
	mp.add_timeout(1, function()
	if tonumber(mp.get_property("playlist-pos-1"))>0 and mp.get_property("playlist-pos-1")~=mp.get_property("playlist-count") then
		nextIndex = tonumber(mp.get_property("playlist-pos")) + 1
		local nextFile = mp.get_property("playlist/"..tostring(nextIndex).."/filename")
		if nextFile and nextFile:find("://", 0, false) then
			mp.register_event("log-message", listener)
			mp.command_native_async({
				name="subprocess", 
				args = {"C:/mpv/youtube-dl.exe", "-f", "bestvideo", "--no-part", "-o", cachePath.."/%(title)s.mkv", nextFile},
				playback_only = false
			}, function() end)
			mp.command_native_async({
				name="subprocess", 
				args = {"C:/mpv/youtube-dl.exe", "-q", "-f", "bestaudio", "--no-part", "-o", cachePath.."/%(title)s.webm", nextFile},
				playback_only = false
			}, function() end)
			mp.command_native_async({
				name="subprocess", 
				args = {"youtube-dl", "-q", "--skip-download", "--sub-lang", "en", "--write-sub", "-o", cachePath.."/%(title)s", nextFile},
				playback_only = false
			}, function() end)		
		end
	end
	end)
end

local function clearCache()
	if package.config:sub(1,1) ~= '/' then
		os.execute('rd /s/q "'..cachePath..'"')
	else
		os.execute('rm -rd "'..cachePath..'"')
	end
end
local f=io.open(cachePath,"r")
if f~=nil then 
	io.close(f)
	clearCache()
end

mp.observe_property("playlist-count", "number", function() 
	if observeInitial then
		DL()
	else
		observeInitial = true
	end
end)
mp.register_event("start-file", DL)
mp.register_event("shutdown", clearCache)