I want to unique lookup from another file and multiple sheet. However, if the data contain in Sheet A while the data range in SheetA contains 400.000+ rows, so the function can’t lookup. The formula below.

=if(I8="","", iferror(iferror(UNIQUE(FILTER(
          IMPORTRANGE("Gsheet url", "SheetA!L2:L"),
          IMPORTRANGE("Gsheet url", "SheetA!B2:B") =I8 
        )),
   UNIQUE(FILTER(
            IMPORTRANGE("Gsheet url", "SheetB!K2:K"),
            IMPORTRANGE("Gsheet url", "SheetB!B2:B") = I8 
         ))),
      UNIQUE(FILTER(
               IMPORTRANGE("Gsheet url", "SheetC!J2:J"),
               IMPORTRANGE("Gsheet url", "SheetC!B2:B") = I8 
            ))))