If you have too many nodes/meshes in your scene, this might result in slow performance. You can check your Geom count in PStats, or by calling NodePath.analyze() on your object or scene.

Panda3D provides a function to reduce the mesh count: NodePath.flattenStrong(). This will reduce the NodePath to only one node. Be careful with this function: Usage without care might cause your game to crash, since you will not be able to move individual subnodes of a flattened node around anymore. Also note that flattening your whole world like this is a bad idea, as you will break culling, which will cause your whole world to be rendered even if your camera is rendering only a small part of it. You will need to find a balance.

NodePath.flattenMedium() and flattenLight() are not as rigorous NodePath.flattenStrong(), but may be worth considering.

Though, if you have multiple independently-moving rigid nodes, the flattening functions might not suit your needs, because, since the flattening functions flatten everything to one node you won't be able to move individual sub-nodes around. An alternative is the Rigid Body Combiner, which can combine multiple nodes while you can still change the transforms on the sub-nodes.

If you are using the GeoMipTerrain for terrain rendering, that might also result in a large mesh count. (You can check the block count by calling terrain.getRoot().analyze().) If it is too high, try increasing the block size, or enable AutoFlattening, which will reduce the block count to only one. The autoflatten function was created because normally you can't flatten a terrain using the normal flattenX methods, because this will interfere with the GeoMipTerrain's updating system.