I don't have much experience working with TFT screens; however I have seen some threads regarding the screen you are using. I will recommend you a couple of them. They talk about SPI speed improvement. You might find them interesting:
Regarding the Arduino libraries question, if you want to use Arduino libraries that aren't currently supported, my suggestion would be to try writing them by yourself. The libraries are usually written according to the hardware architecture, and the Arduino architecture is different to the Galileo architecture.