Faster way to execute loops - excel

Question

I am new to VBA and need help in figuring out a faster way to execute my code. Here is the code I am using:
Sub loop()
For i = 1 To 100000
check_cell = Sheets("Sheet1").Range("I" & i)
For j = 1 To 14430
text_to_check = Sheets("Sheet2").Range("D" & j)
text_to_fill = Sheets("Sheet2").Range("E" & j)
If InStr(check_cell, text_to_check) Then
Sheets("Sheet1").Range("J" & i).Value = text_to_fill
End If
Next j
Next i
End Sub
I know that I am using a very brutal way by running the system through a loop 1,443,000,000 times. Any help on shortening this would be appreciated. Thanks.
EDIT: Based on suggestion, I tried with new code using variants but nothing seems to be happening. Would you be able to tell me what am I doing wrong here? Thanks
Sub loop_2()
Dim varray_1 As Variant
Dim varray_2 As Variant
Dim i As Long
Dim j As Long
varray_1 = Sheets("L1").Range("I2:I39997").Value
varray_2 = Sheets("Sheet2").Range("G1:G14394").Value
For i = UBound(varray_1, 1) To LBound(varray_1, 1) Step -1
For j = UBound(varray_2, 1) To LBound(varray_2, 1) Step -1
If varray_1(i, 1) = varray_2(j, 1) Then
Sheets("L1").Range("L" & i).Value = Sheets("Sheet2").Range("H" & j).Value
End If
Next j
Next i
End Sub

Answer 1

I haven't tested this code, but it should at least give an idea of how to put the values into arrays, process everything "in-memory", and then write the results out.
Sub loop()
Dim i As Long
Dim j As Long
Dim check_cell() As Variant
Dim result() As Variant
Dim text_to_check() As Variant
Dim text_to_fill() As Variant
check_cell = Sheets("Sheet1").Range("I1:I100000").Value
result = Sheets("Sheet1").Range("J1:J100000").Value
text_to_check = Sheets("Sheet2").Range("D1:D14430").Value
text_to_fill = Sheets("Sheet2").Range("E1:E14430").Value
For i = 1 To 100000
For j = 1 To 14430
If InStr(check_cell(i, 1), text_to_check(j, 1)) Then
result(i, 1) = text_to_fill(j, 1)
If i = 1 Then
Debug.Print "check_cell=" & check_cell(i, 1)
Debug.Print "j=" & j
Debug.Print "text_to_check=" & text_to_check(j, 1)
Debug.Print "text_to_fill=" & text_to_fill(j, 1)
End If
' exit as soon as first match is made
Exit For
End If
Next j
Next i
Sheets("Sheet1").Range("J1:J100000").Value = result
End Sub

Answer 2

The most cost here is using InStr(), but you should also:
Declare your variables, Variant are slower
Wrap your loops with
With Sheets("Sheet1")
...
End With
Change cell addressing to .Cell(10, i) instead of .Range("D" & j)
My tests show it runs 50% faster, note that I had all cells empty, so the InStr() cost is relatively low in that case.