One of the things that glcd lib does, which is what makes the rendering code, especially for text
so complex is that it will avoid reads of display memory whenever possible.
For example, when rendering, it renders or paints pixels into a local 1 byte register
that is used as a 1 byte page buffer and then writes that byte to page memory.
Whenever the size of what is to be rendered is a multiple of the page size (8 pixels)
and aligns on a page boundary (8 pixels) then there will be no reads.
When clearing the screen, there are no reads from memory just writes to the glcd memory.
There will also be no reads from glcd/frame-buffer memory if rendering text that is 8, 16, 24, 32, ...
pixels in height that land on a vertical modulo 8 pixel boundary.
Nor would there be any reads from memory to render a bitmap that is a multiple of 8 pixels in height
and lands on a vertical module 8 pixel boundary.
While this is very fast and works great to eliminate reads, this rendering method only works
well for devices that use 8 pixel vertical pages. It also doesn't allow doing things like changing the x,y origin,
rotating the display orientation, or rotating the actual text.
In the big picture, once you get much beyond 4-5 FPS, it's pretty much ok since
the liquid crystal in the display often takes a while to change anyway.
The biggest thing is does the user notice any lag in the display being updated
or does it slow down the CPU to prevent it from getting its other tasks done.
--- bill