There are both modes available. Memory mapped one has lower resolution due to high memory requirements. Default full size one is unbuffered - data is transferred to LCD when drawing however it is not over SPI, there is 8bit parallel bus. See https://banglejs.com/reference#l_Bangle_setLCDMode
Is the graphics frame buffer on the Bangle memory-mapped and accessible from within JS? Or is data transferred in chunks to the LCD controller via SPI?