This interests me. Let's say I have a workbook with two sheets named Main and Index. Main has 50 000 rows of data, and in column 5 of each row is a VLOOKUP pointing to Index. My usual VLOOKUP formula would go something like this: "=VLOOKUP(RC1,Index!C1:C3,3,0)". (I use R1C1 references by habit.) Now, let's take two examples:

1) Index has 15 rows. Would it make the lookups go noticeably faster to say "=VLOOKUP(RC1,Index!R2C1:R16C3,3,0)"?

2) Index has 20 000 rows. Would it make the lookups go noticeably faster to say "=VLOOKUP(RC1,Index!R2C1:R20000C3,3,0)"?

If Excel notices the last-used cell in Index, then I don't see why it would make a difference.