How do I send part of an ArrayBuffer to SPI

That's the main one and you should see some decent speed improvements with that.

The other one would be to move to DMA. If you reserved 2 LCD_WIDTH length buffers then you could:

• SPI write immediately on the first pixel received (or if doing fillrect)
  
• If in progress, write subsequent pixels (on the same line) int the second buffer
  
• When the first DMA finishes, kick off DMA for buffer #2
  
  

I believe this is the sort of thing @fanoush already tried when he ported to some watches with SPI displays - you can effectively be doing the transmission at the same time as working out the next stuff to draw.