On the first frame of simulation (or whenever TP brings Max objects into the system for use), TP has to rebuild its material tree to accommodate the new object(s). When importing lots of objects (e.g. via Obj2Particle or LayerToParticle) this can be fairly slow.
The suspected reason for this delay is the presence of bitmaps in the materials. These seem to cause Max to do a force-check if the texture exists.
If we apply a simple Standard Material with no bitmaps onto the objects, then everything imports into TP much quicker. However, the problem with this is then none of the faces in TP will retain their material IDs (aka "MatIDs"), and then you cannot render the materials properly, nor can you do operations like with SurfacePosition where we create particles only on specific MatIDs.
The solution is a tool we are working on that strips the bitmaps out of all the materials in a selection. Then import those objects into TP for use and it will be faster. However, we are quite busy so the arrival of this tool may take more time. Perhaps you have some free time to write one? :)