It disables ability to change the video speed and it changes the loudness of the video, increasing it. In my case I have to keep the youtube loudness low as well as my pc loudness low in order for it not to be too loud, but the height of the visualisation bars is dependent on the loudness of the youtube playback, to they are very low for me, barely showing above the line.
I may suggest that after you let me change the speed, you 1st & foremost make the size of bars independent of the set video loudness. Then changing the loudness of the videos may become acceptable, especially if it happens to make them all the same loudness, so long as it's not at the cost of dynamic range of the audio.

And all of this should be mentioned in description of the addon as its features, as well.

edit: oh, it's an old addon and youtube has likely changed a bit in that time.