Can you clarify what you mean by
“Can anyone guess why the pyro takes so much time to render out and doesn’t properly give spritesheet while doing mosaic output ?”
If I understand right, it sounds like the image is also massive. if you’re renders are 256x256 and you have a 8x8. your image will be 8x512 sq in pixels = 4096.
If that’s the case, throw a ‘scale’ node down after and set the actual size of your final sprite sheet.
I would export different passes of your pyro using the mantra image planes. Then in comp split the .exr file, do any tweaks and then pass it to a mosaic node.
*side note - If you render out a full sequence, before you throw down the mosaic node you can do a timeswarp . On frame one set the value to what you want the first image to be in the mosaic. Set another / last key on the last frame number you want in your texture sheet. Set the value for the second key to the frame in your image sequence.
In this instance if you’re asked to see more of the ending or the beginning, you can change the curves in the animation graph or the values on the frames; rather than trying to re-render everything.