VBA Macro to generate a report of mean values for each date
In my example the iteration needs 2 seconds. How long is your code running?

Note 1:
Reduce the number of iterations. You are iterating 100.000 times

Note 2:
You are accessing the same cell Cells(l, 2).Value very often in the second for loop. But the value does not change. It will change in the first for loop.

You can save the value in a local variable in the first loop:

Dim l2 As Variant
l2 = Cells(l, 2).Value


Note 3:
You are accessing the same 90 cells Cells(1, m).Value very often too (1000 times).

You can save the 90 values in an array before looping:

Dim Arr() As Variant
Arr = Range("K1:CV1")


My complete code looks like this:

' Mean Value
Dim Arr() As Variant
Arr = Range("K1:CV1")
Dim l As Integer, m As Integer
For l = 2 To 1000
    Dim l2 As Variant
    l2 = Cells(l, 2).Value
    For m = 1 To 90
        If l2 = Arr(1, m) Then
            Cells(l, m).Value = Cells(l, 5).Value
        End If
    Next m
Next l