During the fetching o data that occurs in the following function:

main.py:

# Aplication.onSubmit:
def onSubmit(self, entry: GUI.QueryEntry, event: Event):
        search = self.search['Search']
        search.content.clear()

        if not entry._place_holder:
            try:
                vid = API.parseURL(entry.entry.get())
            except Exception as e:
                print(e)
                return
            
            videos = API.parseVideos(vid)
            temp = lambda v,e: threading.Thread(name="OnClickVideo", target=self.onClick, args=(v,e)).start()
            API.populate(videos, self.videos, Video, self.after, search.content.addItem, 6, offset=(0,5), bind=(["<1>"], temp))

This block of code do a lot of networking with requests and threads , the slow down on the GUI could be due to the main thread having to keep jumping between GUI's calls during the rendering of the fetched data. If that's the case, we need to somehow make the GUI's calls be executed in different threads, not only the main thread on Tk.mainloop.

Streams with mime_type=='video/mp4' that aren't progressive don't have sound due to this reason.

Meaning that our only work around would be downloading the 'audio/mp4' and the 'video/mp4' streams separately and somehow putting those two together, a quick and easy way to do this is using moviepy but performance wise this would be a disaster a 20+ minutes video would that almost 30 minutes to finish rendering.

To see how big of a issue this is, take a look at this random video I fetched with pytube:

progressives:

    <Stream: itag="18" mime_type="video/mp4" res="360p" fps="30fps" vcodec="avc1.42001E" acodec="mp4a.40.2" progressive="True" type="video">
    <Stream: itag="22" mime_type="video/mp4" res="720p" fps="30fps" vcodec="avc1.64001F" acodec="mp4a.40.2" progressive="True" type="video">

non_progressives:

    <Stream: itag="160" mime_type="video/mp4" res="144p" fps="30fps" vcodec="avc1.4d400c" progressive="False" type="video">
    <Stream: itag="133" mime_type="video/mp4" res="240p" fps="30fps" vcodec="avc1.4d4015" progressive="False" type="video">
    <Stream: itag="134" mime_type="video/mp4" res="360p" fps="30fps" vcodec="avc1.4d401e" progressive="False" type="video">
    <Stream: itag="135" mime_type="video/mp4" res="480p" fps="30fps" vcodec="avc1.4d401f" progressive="False" type="video">
    <Stream: itag="298" mime_type="video/mp4" res="720p" fps="60fps" vcodec="avc1.4d4020" progressive="False" type="video">
    <Stream: itag="299" mime_type="video/mp4" res="1080p" fps="60fps" vcodec="avc1.64002a" progressive="False" type="video">

In this scenario, the only resolutions available with audio for this video are 360p and 720p. It was lost more than half of the available options.