So I have this restaurant and my host is at max level everything. But clients keep piling up at the entrance and she takes frickin' forever to seat them. I even used a mod to hire one additional host and bought one additional host stand, but one of them just keep standing there like an idiot while the other do all the work (sloooowly).
My PC is not great (it fits the requirements tho), I have some times where sims take a bit longer to respond to commands, but still, this is ridiculous. I can't make a proffit because my clients don't get seated in time.
The whole host idea is stupid to begin with. Most places I dine you just go in and take a seat. So it should be optional. But I found no mod to get rid of them.

Besides the fact that restaurants were always super buggy, you should make sure the sitting area is close to the host station (and the kitchen), and it's best to avoid having things on different floors.
also make sure that you didn't use MoveObjects on any of the functional objects in the restaurant (should be a general rule as well, never use MoveObjects on functional objects).

Hey, that actually helped a bit (putting the host in the same room as the tables) thanks.