Manual check outs are absolutely faster than someone self scanning, mainly because the self scan software is not optimised for speed but for not confusing the customer and lack of loss (weighing every product.) but the main reason is that the person operating the machine is someone who is trained to do that and they do it all day long, so even the worst checkout operator is going to be faster than the average SCO user.
But... you can put in 10 self checkouts in the space of 3 manned tills, therefore you can be serving 10 people at the same time so it "looks" like self check out is quicker as the queue goes down quicker, every few seconds someone is moving forward. But of course this is just perception. And usually as well, out of the three manned tills there might only be one open. Gone are the days where you'd have 30+ tills at large Tescos all running at once.
And of course, most people struggle on SCO as they're not used to finding barcodes, reading the screen, weighing the product, clearing errors etc. However, as someone who used to work in retail I can absolutely scan faster than the SCO can keep up (because it has to weigh each item etc) and if I was on an actual till I could just chuck it down the chute being even faster. The main delays on manned checkouts are waiting for customers to get their money out, have a chat etc.