Unfortunately I can't replicate the behaviour you're seeing on my own device; I tested with the transform in your screenshot with a loop over 25 imagebuttons.
That said, what I do suspect - are you using imagebuttons that are the size of the full screen + focus_mask True to have only the hover area be the opaque part of the image? This is known to cause a lot of performance issues (independent of the outline shader) due to the cost of the calculations Ren'Py runs to test for pixel transparency when hovering over an imagebutton. If you have several full-size images on top of each other, Ren'Py is running calculations on the transparency of the pixels of all of them. To avoid this, I suggest you cut down the transparency to only be a little larger than the button image itself (the padding is so that the outlines will still work nicely) and position it in-game with xpos/ypos.