Overlay + Multiply Filter, very slow, can it be sped up?

[-Corso->]

Hi All, just wondering if there is anything to speed up this process?
Condensed version. I've made the full day cycle for the Cairo Test World. I really like the sunsets and sunrises over the mashed terrain currently (complete with moving shadows). BUT, the sun effects take a fairly long time to generate on the output page.
I have to apply the sun overlay colour and the multiply black, every single screen move (during sunrise and sunset).
Long story even shorter.
1. Generate Screen (the whole picture).
2. Apply Sun Colour 'Overlay'.
3. Apply Black 'Multiply' for nighttime fade in.

Is there per chance some other way, even non-Cairo of applying fast filters? Or is that not possible?