Importing Sprites is really fast, so they are probably getting imported fine. For the 10 seconds, I guess you are not using an SSD, so I/O operations just have to take their time.
BMP is just a map where each pixel is represented in a matrix, a number of times for the number of channels. There is no compression. So unless your png changed size in pixels, the converted bmp will have the same size in bytes.
BMP is just a map where each pixel is represented in a matrix, a number of times for the number of channels. There is no compression. So unless your png changed size in pixels, the converted bmp will have the same size in bytes.