How do I send part of an ArrayBuffer to SPI

This optimization is perfect for drawing horizontal lines

Display 240x240(320)

before: 990ms, after 266ms

hLines = () =>{
      g.clear().setColor(0xfffff);
      xm = 240;
      ym = 240;
      t1 = Date.now();
      for ( var y = 0; y <= ym; y+=10){
          g.drawLine(0,y,xm,y);
      }
      t2 = Date.now()-t1;
      g.setColor(0xff0).setFont("6x8",2).drawString(t2.toFixed()+"ms",0,0,true);
};

I guess it should be possible to optimize this for vertical line too?