Is being a waitress hard work ?

Yes it's hard.  

My best advice is:

1)  Know that you will make mistakes.   It's ok, everyone does especially at first; and

2)  Try to remember the 90% of customers who are decent, polite human beings who appreciate the service you provide rather than the 10% of customers who are vile pigs who will treat you like shiit on their shoe.

very hard work if you hate people and are slow. 

Easy for most. Can you walk? Take food to people?

If working mostly for gratuities, as in the USA, your goal is to be well above Federal minimum wage, and for that, you need to be able to do an outstanding job. Some businesses cheat their employees having you work extra at minimal pay. I don't know whether your employer is fair or cheating. Yes, as a server you have to know the menu and what the kitchen is capable of, get orders accurately and served to good timing. You should not have washing dishes or mopping floors. It's bus tables, no "t".

You do nor state your location and a waitress where you are may be harder than here. Done correctly, it is a semi-skilled job of reasonable pay scale.

If the job as too many responsibilities, then it is not good.