I have not served in about a year, but do manage and see that whether you are a on lunch or visit a restaurant during "lunch time", guests think that it's ok to TIP less.
Let me explain what happens in most restaurants during lunch time...
Most restaurants do their best, because management knows you have 30 minutes to an hour until you have to get back to work.
You are getting the same quality of service, while servers and especially cooks are supposed to be busting their ass to make sure you eat and get out in time.
I must say, there are times a ticket is missed or something may take longer (well done burger). Most of the time if this happens you get a comped meal.
Back to the subject, I don't understand why it's ok for guests to tip their server less than 20% just because it's lunch. Just my observation and opinion.
On that same note, many restaurants have specials for lunch. For Example; 1/2 price burgers or just reduced prices on something on a certain day of the week. Many servers will put the original receipt in the check presenter to show the guests "what their bill should have been". Many guests tend to just tip on the discounted amount.
I'd like the opinion of guests on what they think when they go out to eat and how they base what they are going to tip.