Bad performance after painting weights

Hi,
I’ve created models with Mixamo Fuse and it works good. I have constant 60 FPS but… these models are not ideal (pervading meshes etc.) so I’ve edited weights in 3ds Max. After that performance down to 30 FPS… Moreover I’ ve noticed similar problem with my horse bought in Unity Asset Store but all other models works good… I have only problem with that horse and models with edited weights… Do you have any idea why?

If you have any questions I would be happy to answer. Please help me.

Thank you.