I see that you are referencing a different file - can you copy the
reference sheet into the workbook that contains the lookup formula?
That way the formula will be a lot shorter and less complex. The sheet
could be protected and hidden if you don't want Users to tamper with
it.

If you are stuck with it as a separate file, you might like to define
named ranges within the reference file, i.e."T_1" referring to
$J$7:$K$18900 and "T_2" referring to $H$7:$I$18900 - again, your
formula will be shorter.

I assume that you are copying the formula down a number of rows, so
obviously the formula itself will take up memory, as well as the value
it produces, so anything to reduce the length of the formula will help
your cause.

Might help this time ...

Pete