Derailleur tension causing chain friction? I have a foldie which recently seemed a bit harder to pedal. Specifically back pedaling seems harder than it should be (if I kick the pedal back, it barely travels a quarter or half turn. I noticed that if I push the bottom jockey wheel up a little, reducing the tension on the wheel, the backpedal test results in 2-4 times as much travel. This problem is also only pronounced on the lower (bigger sprockets) gears. I recently detached and reattached the RD, to install a derailleur guard on the wheel, and subjectively it has seemed different from that time, though objectively I have checked a few times and it seems installed correctly. I also have a squeaky chain lately which I believe means it needs to be lubed, which I am going to do over the weekend. 

TLDR: Does the dry chain explain why the back pedaling seems so bad? Is it a matter of lubing the RD jockey wheels? Or is it some other issue. I also see some rust on the RD springs, not sure if it has any effect.
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.