excel works slowly when filter

Hello guys,

I have a problem with an excel file and I cannot find a solution.
So I have a sheet with 3500 rows and a macro that completes columns AG,AI and AK based on value from column D.Macro will launch every time a value in column D is changed (I'm using function Worksheet_Change(ByVal Target As Range) ).
Everything works perfect except when I filter the rows. When I filter by an unique value (so only a row remains in my selection), the macro works very slow. And not only the macro, even if I try to select that row, the excel moves slowly.
If I filter by a multiple value (for example remains 5-6 rows), only on the last row or last two rows macro and selection works slowly.
I'm not an expert in excel and I cannot find the problem. Why an excel would work slowly only on filtering and only on some specific rows? That make sense for somebody?

Thanks in advance
Marius

Ok, so I've uploaded an example.
First case is when the filer for column B(FG) is not set and all the rows are shown. So if I write a value in column D, for example, you will see that columns AG,AI and AK completes automatically based on a macro.
The second case is when I filer column B.So I go to text filter, select contains and write for example BARK5803100P-LF-M02 . Now if I change value in column D, the columns AG,AI and AK will update very slow.
The third case is if I filter column B for contains lets say BARK5.Now if I change the value from column D for the first rows, everything works perfect. But if I change it for the last row, it works very slow again. The same thing even if I try to select some cells from that row(so no macro implied).

I hope that I was clear.
Thanks again

Yes, I have. But why the excel works slowly only on the last row? And how can overcome this problem without unmerge?