Foreign workers (slaves) work at half the speed of natives. The big advantage of the foreign workers is that they are FREE. You will never have to pay support for them.
When I have successful wars, I eventually phase out my own workers (since I have to pay them) and rely entirely on slaves.
I move my workers around in large groups so I keep the foreign workers in pairs so that they are easier to count. A typical "work group" of mine is the strength of four natives. This might be any combination, 8 foreigners, 6f+1n, 4f+2n, 2f+3n, or 4 natives. Makes it easy to keep track of how long road-building, mining, etc., takes.
When a power gets wiped out, I sometimes join slaves of their nationality to my cities. I never join foreigners to my cities when their home country is still in existence.