code slows down when forced to handle massive data

I am sitting in front of some vba codes I have written with a friend of mine. We have a lot of data and are stucked with the performance. Maybe one of you can help us.

We are running a macro mainly based on array formulas with a lot of references a look ups to different cells. For example we have this code, which runs quite fast ( ~ 20 sec.) with 10k data but slows down to > 4 min. with 30k data. Does anyone have an idea what the problem is or how to solve?

I hope we have written the code appropriate:

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At first glance I would say set this to false at the beginning of the code. You have it set to true at the start AND at the end, should be false at the start.
Application.ScreenUpdating = True

It will go a lot faster.