So I'm at the shop trying to figure out why a relatively recent Ultegra drivetrain is shifting so poorly. The rear derailleur in particular is stiff/sticky.

So I take it apart - as in every part disassembled and put back together. The diagnosis is that every microgram of grease/lubricant that once inhabited this little gem is completely gone. Blown out, washed out: whatever. Someone probably saw the need to marinate in a bath of degreaser followed by some compressed air.

I know of no other shop or individual that would have the patience or experience to take apart a Shimano rear derailleur. Most shops would say that the drivetrain is 'worn out' and hand the client a $1,000 tab for a new drivetrain.

Degreasers: do NOT use around any parts that require grease lubrication, such as hubs, headsets, bottom brackets or derailleurs. And when you do actually need a degreaser, that water-based degreasers are fundamentally useless, which includes anything 'green', 'bio', 'organic', etc.