With many tables to process, Word is probably not getting enough breathing space for its housekeeping. You're also unecessarily selecting each table, which only adds to the processing overheads. Try:
Code:
Sub Tabellen_formatieren()
Application.ScreenUpdating = False
Dim i As Long
With ActiveDocument
For i = 1 To .Tables.Count
With .Tables(i)
With .Range.Font
.Name = "Arial"
.Size = 10
.ColorIndex = wdBlack
End With
.PreferredWidthType = wdPreferredWidthPercent
.PreferredWidth = 100
End With
If i Mod 25 = 0 Then DoEvents
Next
End With
Application.ScreenUpdating = True
End Sub