Fast mesh creator for Unity that allows to calculate mesh data separately in other thread.
Go to Releases and download the archive. Then unpack the archive into your project's assets folder.
// You can put this in other thread.
MeshData meshData = new MeshData(verticies, triangles, normals, tangents, colors, uv);
// This you can put in main thread.
Mesh mesh = MeshCreator.CreateMesh(meshData);