It sounds like you have your lights with the “Cast Shadows” option turned on. This is a misnomer for Krakatoa since particles will always self-shadow. The “Cast Shadows” option enables shadow casting from geometry objects onto the particles. This would slow down the lighting calculation and may not be what you intended, turning that off for your lights will improve performance.
On the other hand, we generally expect reasonable performance from the matte object rendering. Assuming you aren’t using extremely heavy geometry objects, would you be able to send the scene (or a similar one) to krakatoa-support@franticfilms.com so I can figure out what is causing the slowdown?
Thanks,
Darcy