I have had similar experience. I it once took me 4 days to free up a sluggish 201; to start with it was pretty clean, very dry and hardly any dried up oil residue. Still something made it sluggins, and it wasn't until I had oiled and test sown 4 days in a row it suddenly freed up. I guess a 401 with sticky old oil can be even more of a challenge, more gears and parts to clean and flush out dried up oil from. It can take days to dissolve stubborn grime. A straight stitcher like the 301 might run faster with the same motor power, but the 401 is pretty fast when cleaned and finetuned.
I keep up a daily oil and test running routing on a machine like this for a while. Detecting all oil points can take some time on a new model too. I once had machine that turned out to have the needle bar go up and down with quite a bit of resistance. Even after thorough cleaning, oiling and test sewing it was slow, I didn't realise until after yet another turn of cleaning and oiling. I poked around with a tooth pick and it looked like sticky grime had turned up where the needle bar moved up and down in the body. Heaps of oil down there did the trick. It was a bit messy,and required a bit of clean up, but it has been fine since. My point is; you don't always know which link(s) are frozen or sluggish, so keep oiling all of them. Some points are hard to reach, and needs extra attention to get oil all the way.