Picture symbology is a processor intensive task. This is especially true for alpha channel PNG.
To enhance the speed, we recommend using images as small as possible. For example, if you plan to never render symbols larger than 64 pixels, use images of a corresponding size. Using much larger images will slow down rendering significantly. Rendering a 64x64 image requires processing (with Alpha channel) only 16KB whereas rendering a 512x512 image requires processing 1MB, making the processing even 64 times slower!