overall slowdown for dense drawing.

I thought I had gotten around this problem , however I find there is a limit to how large a file can be before the eraser , brush and other tools slow down radically ( IMAC 4 GIG.)
I have a single cell which is drawn beyond the camera box . This cell is then pulled past the camera ( using motion/screenplanning ) to replicate motion.
The cell is four times the length of the camera box .
I optimized and flattened / saved the drawing as I went along but I am still at a point now where even a few drawn lines takes seconds to show.

Any work arounds ? Other than breaking the drawing down into individual parts and then pasting the finished movies together as one ?

The drawing depicts a train of letters ( a.b.c.d…) being drawn by a tractor , each letter being a sketch in itself.

So it could be done piece by piece , if that’s the only way.