why function taking table parameter runs so slow?

Hi,

fake Codes likely:

let
     table1 = sometableInSameQuery,
     table2 = anotherTableInSameQuery,
     #"Added Conditional Column" = Table.AddColumn(table2, "new column", each if [C1] = null then table1{[C1=[C2]]}[C3] else [C1])
in
    #"Added Conditional Column"

table2 has 5000+ rows, table1 has 800+ rows, content of table1 is gotten from JIRA REST API.

when apply changes, I found both table1 and table2 can be loaded very quick.  But the new query loaded very very slow. Why? I am wondering if table as function parameter consuming resource too much? Or does JIRA REST querying need every time when Table.AddColumn executed row by row?