You have three systems involved and unless you isolate them you're just blowing smoke. To isolate the actual problem you'll have to use a divide and conquer method.

First spin the wheel with your fingers on the top loop of the chain and see if the cassette is trying to spin and feed the chain. You can also test this by removing the wheel, and installing with the chain off. Spin the wheel and hold the cassette to test for freehub drag. If you have freehub drag, donm't take it apart just yet. Look behind the cassette, or remove it and check if a bit of road debris or piece of string is wound up back there binding the cassette to the hub shell.

Next drop the chain inside the inner chainring and park it on the bottom bracket shell. You might need to use a piece of cord to hold it clear of the chainrings. Now spin your cranks and both watch and listen. They should spin freely and be silent or have a smooth whirr kind of sound.

Now put it all together and check the chain for internal friction by back pedaling slowly in a middle gear with your hand pushing the lower RD pulley forward slightly to slacken the lower loop so it sags a few inches. Feel the RD cage for any jerkiness as a stiff link or section of chain spools through. Also watch the chain spooling into the lower loop for any links that don't straighten by gravity alone.

If all the above are OK, it's possible that your RD pulleys are stiff, but I doubt that's your problem.

Also be aware that some chain lubes can thicken or harden increasing the parasitic drag within the chain causing greater internal friction. Just on a hunch you might dry clean and re-lube the chain and see if that solves the problem.