I only checked beginner's sb and taiko's sb. You should make sure the fixes are made everywhere.
[beginner]
26195 - Here you can fade out Okami Night BG (SB) because it's being covered up. Just because you're not "doing" anything with it, doesn't mean it's not being drawn. Fade it back in when you use it again.
[taiko]
First, the taiko backgrounds are stored as jpg. This is a good thing for pictures, but it's very easy to see the compression artifacts on the text (where something like png is suited better). If you still have the original images, I recommend you re-make them all and then storyboard in the text which would be saved ideally as a png file.
The "yuugure" and other icons that just show up, well clearly I'm not seeing the point. For one, they should just be part of the background unless you're doing something with them, or you want to minimize the compression artifacts. The other problem is that you used a blur for a lot of the background, which works well except when the canvas runs out. To make sure that blurs look good, you need to make sure there's at least 1 completely transparent pixel around the image. It's very easy to see the line for the "yoru" and the "shinkou" images.
[periera]
00:14:464 (1) - consider deleting this note whenever you have a stream of 4 or 5. traditional taiko rarely has groups of 4 or 5, and is mostly 3.
00:20:111 (2) - ^ and every instance afterwards.
00:21:699 (2) - clap
01:17:817 (5) - whistle
[winber]
just nazis
01:34:405 (3) - for example