I have a very simple scene with just 30 sprites and a 512 * 512 texture, and Mantra takes about 30 seconds before it start rendering, on ever frame of a sequence.
I tried rendering a totally empty scene, Micropolygone, no textures, no headlight, no geometry. Always half a minute delay before the rendering. So 25 black frames frender for 10 minutes.
I´m using Houdini Apprentice HD 11.1.22 on a 2011 Macbook Pro. Mac OS 10.6.7
You can start a Houdini terminal/shell, launch Houdini, go to the Mantra ROP's Properties > Statistics tab and set the Verbose Level to 3/5. Now, you can try to render and see what is printed in the terminal to get an idea of what Houdini is processing before rendering starts.