Use native plugin to do heavy job?

Is DllImport (“__Internal”) efficient enough to be used to de heavy jobs?

For example, i have a data compress algorithm written in C, will i get a not bad result if call this as native plugin?

Per the Unity plugin documentation:

Managed-to-unmanaged calls are quite processor intensive on iOS. Try to avoid calling multiple native methods per frame.

As long as you’re not calling these functions multiple times during a FPS cycle, you should be fine. Doing heavy calculations, as long as it’s either within the confines of Unity or iOS or whatever platform you’re targeting, should be just like any other processing. Once again, the key here is not calling it excessive amounts.