The Louvre has been one of the most inconvenient places to eat we found. Last trip the cafeteria which has pretty decent food was literally out of every entry we wanted, they had one thing left which we didn't want. This is a place where you should be able to get a tray of food and sit down -- if only they had the food.
WE took our granddaughter to the restaurant which as I recall is to the left of the coat check and the food was good and prices not crazy but they define slow. We were not expecting a fast lunch but it was truly excruciatingly slow. The people next to us had ordered some pasta thing and finally left in a huff as impeccably dressed waiters cruised around the room fiddling with napkins and being busy but never actually coming to customers and making sure they had orders taken or food delivered. (the couple was French so not just an impatient American tourist.) We were tired so a long rest was fine -- but sooo slow -- I think we waited about 20 minutes even to order while these black clad waiters cruised around avoiding eye contact with patrons and looking busy.
You can get sandwiches at a counter on the first floor of the pyramid and pastries and sometimes ice cream; you then search for a seat around the pyramid area. They are designed to perch on and they slope so carefully designed for people to eat and run and not lounge around. Many people end up on the floor eating their sandwiches as there is often no convenient spot to perch.