Lower driving speed make Carbon Attaching to Engine surface?

Low rpm driving will definitely help conserve gas and help the environment. There is nothing wrong if you are driving Japanese make models. However, if you are driving German products, their design is different such that it requires high heat on interval basis to clear up the exhaust (not the engine surface) system. High temp. can burn the unburn carbon residue and vaporize it and avoid eventual build up in the exhaust so your car can run better in the long run.

You don't need to drive fast to achieve high rpm, one way to do it is to switch to sports mode (if your car is so equipped) or down shift to a lower gear but maintain the same speed. That will kick the rpm high (and consume more gas) but will not accelarate too much. I think this is far better (and safer) than let the rpm run without engaged to the transmission, which is EXTREMELY DANGEROUS TO DO and should not be performed per owner's manual.

Engine running on high spped without engaging to the transmission is not going to do any good to the engine (and it will cause pollution to the garage too.

Low speed will not achieve high engine temperature. therefore, carbon residue will remain at the exhaust, not the engine surface.