I purchased the 5" TFT from Core Electronics a few months ago and have been working on getting it working. It works on a simple UNO example fine but on a more powerful MCU like the STM32F401RE Nucleo forget it.
After some weeks of effort I have the solution. The Adafruit_RA8875 library is lacking in some areas and does not handle the faster processors at all well. Most graphics functions will work but not bitmap drawing. Also getting an external SD Card to work (on the same SPI port) is finicky at best.
I have created a Github repository (GitHub - dennyem/Arduino) with my solution which consists of a replacement RA8875 library (RA8875_SPI2) which allows an external SD card to be attached to the Nucleo’s SPI 1 port (SD and Fat facilities do not generally allow alternate SPI ports) and using SPI 2 for the TFT. I have also modified the drawPixel() function to work with the faster processor. The SPI2 pins can be configured within the library so it should work with other processors though I haven’t tested it.
The example sketch included also drives the image drawing capabilities to the fullest by sending an entire scan line of pixel data at once. It would be interesting to see if anyone can speed it up further (reader challenge…). In order to use the library it needs to be moved to the Arduino’s “library” path. The images (both 24 bit and 8 bit examples) in the “Data” folder should be copied to an SD card.