Open ff7_opengl.cfg file and find the option to compress textures and turn it to yes. Here's an example
# store external textures in a compressed cache for increased performance
# compression is not lossless, some artifacts may appear when this option is in use
# texture cache does NOT update automatically if the source image changes, however, deleting anything from the cache
# will cause that file to be recreated from the source
compress_textures = yes
If the value after the equal sign is no just erase and make it yes like the one in the example.