Ask the school, how long breakfast and lunch times are.
Then, based on that, you will have an idea about how long they have to eat.
And talk to the Preschool teacher, about it. And HOW they handle it if a child is a slow eater.
Every Preschool, will vary and every Preschool has different routines.
Per your child, you said she is a slow eater.
So like what? Does she take 1 hour to eat a meal?
Does she take 20 minutes?
If she can eat within 20 minutes, I would think that is fine.
At my kids' preschools, they got about 20 minutes for meal time.
For example.
If you can teach your child to eat within a specific timing..... then at home you could use a timer. And "practice" eating with a timer. So then your daughter learns about "timing." At that age, kids do not know how long "20" minutes is. For example. They do not gauge time, at that young age.
So you could get a timer, or show her on a clock, how long she has to eat from start to finish.
ALSO teach her, to know her body's cues. For being "full."
And that is when to stop.
Also, if a child does not eat the entire plate of food, that does not mean they did not eat enough. A kid stops eating, when they are full. Not according to whether or not they ate the ENTIRE plate of food.
So teach her that as well.
I work at a school.
Many kids do not or cannot eat, EVERYTHING on a plate.