Derailleur tension causing chain friction?
An un-lubricated chain will definitely cause losses in the drive train. Problems with the jockey wheels may also cause some losses, but probably not as much as the chain

You should properly clean the chain, sprockets and derailleurs and drive train before lubing the chain. There are numerous guides on how to do that on-line that you can easily look up.

Servicing the jockey wheels is reasonable easy and again there are guides available on on-line. The Global Cycling Network YouTube channel has a good  video.

General drive-train wear may also be an issue. so check the chain and rear sprockets for wear when you are cleaning them. There are inexpensive chain wear checking tools available such as Park Tool Company's CC-3.2. Checking sprockets for wear is more difficult as you have to examine sprocket teeth by eye. Worn teeth become asymmetrical and look like curved shark fins. 

You say you disassembled the derailleur. If you disassembled the cage and took the jockey wheels out, is there a chance you swapped them around?  The top and bottom jockey wheels are different.