Flow rate is variable round the flat, I stole a flow cup to test....
The idea is that as a new kitchen is going it, prep everything so that it is not going to be removed for any reason. Hence rewire and adjustment of the drains so they have more drop and all of them have waste traps fitted.
Have you measured the flow rate as any better elsewhere in the house? Tank or mains fed? And if the latter, lead into the house or plastic?