Thanks @vsr1
The query including the predicate is identical. Only difference is that the value for one of the filters is different. e.g. field_x=A for the slower execution & field_x=B for the faster execution.
“field_x” is not a part of index but that is the case for both executions. So we expected that for both executions, the ItemsIn/Out would be same till the Fetch phase. But that is not the case.