Subset. Why is a Unique Append slower when having (C/B)Lobs in your table?

Situation

Using DATPROF Subset to subset a table with a CLOB or BLOB using a Unique Append is significantly slower than subsetting tables that do not contain these data types.

Why is that?

Explanation

DATPROF Subset adds the data to a table with unique append using a join to the Source table via the databaselink.

Oracle however, does NOT support the use of join through a databaselink on tables with Clobs or Blobs.

For that reason Subset first copies the whole source table to the Target environment.

When this table exist in the target it is used for the unique-append-join and then dropped.