full-screen background image

In recent builds I have added a function called drawImages which allows you to draw multiple 'layers' at once, at different scales and rotations: https://github.com/espruino/Espruino/blob/feb45fe7378010be74e8952e087d23950dac050f/libs/graphics/jswrap_graphics.c#L2234

It should go a long way towards making this much easier.

Right now, the 'Image Clock' app exists which does a fullscreen background: https://github.com/espruino/BangleApps/blob/master/apps/imgclock/app.js

And that works with a digital clock by just copying the area of the image where the clock is to an image stored in flash (The new drawImages would be a better way to handle that though).

In terms of update speed, it's a mix of things -the default path for rendering has to deal with multiple BPP, screen rotation, etc so has a lot of overhead. New firmwares are a lot faster though as there's a 'fast path' for common operations.

In terms of an analog clock - I'd suggest using drawImages. For example:

var cg = Graphics.createArrayBuffer(16,200,1,{msb:true});
var cgimg = {width:cg.getWidth(),height:cg.getHeight(),bpp:1,transparent:0,buffer:cg.buffer};
cg.fillPoly([8,0, 16,100, 8,110, 0,100]);
g.clear(1);
g.drawImages([
  {image:img,x:0,y:24,scale:4},
  {image:cgimg,x:120,y:120,center:true,rotate:0.1},
  {image:cgimg,x:120,y:120,center:true,scale:0.7,rotate:1.1},
],{})

Potentially you could even use a bounding box to redraw only what had changed... For hour/minute drawImages will be fine, but for second hand you may notice some lag from the redraw if you don't attempt to do something to restrict how much gets drawn.