Internalising lot of meshes makes the definition larger and the workers need more time for opening the model which increases overall processing time. Loading models from your Dropbox could be faster but it depends on connection too. Read more about processing time below.
Also check your account limits for maximum Grasshopper definition size in this Documentation article.
The best approach is to model meshes parametrically, your definition will be small in size and can perform better than internalised or referenced static meshes. We have lot of tutorials which can give you tips on how to improve your parametric model:
You can use Rhino ReduceMesh to make the meshes smaller before internalising or referencing them. The command gives you options to reduce the vertex count (hence size) while maintaining acceptable quality of the mesh.