Why is Mcdonald's so slow?

Ok, so for reference the kitchen was ran by 2 people, the chef and a kitchen porter (usually a 16 year old boy.) It's very small and in a pub where we extended to a large dinning room. We usually did allow small customisations like 'no tomatoes' by typing a message in on the till, that wasn't a problem. However, we basically had a night where you could 'build your own burger' by ordering from a large list of options (it was just me working behind the bar). We did this on the busiest day of the week within a two hour slot, when we got over 80 individually customised burger orders because the owner also put the prices down. So the customer had a huge range of choices they could choice from, including: .5 different choices of buns. .Two different types of burgers ie thin or thick patty, you could add up to three burgers on top of each other, .5 different choices of cheese ie swiss cheese, ghouda, melting chilli cheese, edam, . about a choice of 8 different sauces ( you could only have three per burger though) . a huge range of odd veg to add to the burger ie hand cut pickles, kale, baby tomatoes, or regular tomatoes etc... . Different kind of fries is crinkle cut, thin or thick. It would be ok for a real restaurant with a full team, but there were only a chef and a kid in the kitchen, I was only 17 myself, and the till system was so old, it couldnt handle all the customisations and kept going blank, restarting and any order i was entering would be lost. I'd have to chase people around the pub to double check their order. I was only new and if i made a mistake on what type of cheese they wanted, Then we would serve it and the customer would be angry. This created a backlog because the chef would have to remake an order with the specific cheese, buns, veg etc. It was total chaos and confusion. It was something my boss wanted us to try out to see if it could be a regular thing, but we didn't do it again. Any kitchen is chaos, but especially when there was such a huge range of ingredients and it's all put together by hand by one person and a clueless 16 year old boy. You're very kind if you don't mind if a tomato does end up on your burger, try telling a drunk middle aged man we've run out of chilli cheese 😄.