Unfortunately you can't use an attribute sampler mesh as an input to a mesh renderer.
I suppose you're using PopcornFX v1 ?
which version exactly 1.13? for Unity or Unreal?
There might be a way to override the mesh renderer in unreal when you instantiate the effect, but I'm not totally sure about that.
40 meshes isn't that big, so I'm surprised you see a significant freeze on load.
One thing you could try is to bake all your meshes inside a single FBX.
So you'll have 1 FBX with 40 geometries, then create a single mesh descriptor referencing that mesh, right-click on the mesh descriptor and select "Explode submeshes to descriptors", and it'll create the 40 descriptors for each of your sub-meshes
As it is a single physical mesh file, it might be faster to load ?
Hope this helps