i see similar things quoted a lot, but your car doesn't stop when you shift manually, it just loses drive for slightly longer, while you're between gears.
A slower shift loses some momentum and so a little time, but you can't just add up the shift times and get a total lap time loss.

LOL

Same here...,😉

Your analysis is wrong (but a common mistake). The laptime would only be slower by the sum of all the shift times if the car suddenly came to a screeching halt while you shifted and then resumed driving after. But instead, the car only stops accelerating for a fraction of a second during the shift (it doesn't fail to keep rolling at nearly the same speed it was traveling before the shift while you're shifting).

Edit: Sorry, didn't see Footsoldier's (correct) post before I replied...