Use a rs485 communication module

That's really strange... and you're using up to date firmware?

On a 7x7 graphics canvas it should be almost instant.

Could you post up a bit of code as an example that shows the slowdown, without the RS485?

For example:

var g = Graphics.createArrayBuffer(8,7,1);
var t;
t = getTime();
g.drawString("Hello");
console.log("Draw took ",getTime()-t);

t = getTime();
g.setFontDennis8();
g.drawString("Hello");
console.log("Draw took ",getTime()-t);