The loading times by themselves are not the problem. I think the issue is that all mipmaps are created in the same frame, when the plane (with many textures) is rotated into view. Will test and report back.
For reference, this is what fixed the initial problem for me (used in the ITextureEffect apply function):
int[] pixels = ((DataBufferInt)img.getRaster().getDataBuffer()).getData();
System.arraycopy(pixels, 0, dest, 0, pixels.length);
int diff = dest.length - pixels.length;
System.arraycopy(pixels, pixels.length - diff, dest, pixels.length, diff);
Note: You need to be careful that the data buffer of the BufferedImage "img" is actually an int[] array(!)