UPDATE on column referenced in sub-query
Can you create indexes? Are TRANSFORMED_TYPE and/or TRANSFORMED_KEY unique?
The DB hast to compare xxx rows from "where workSet.MARK = 1" with yyy rows from "where source.SELECTABLE = 0 and source.MARK = 1" in subquery(FTS).
So you can calculate yourself: xxx * yyy rows = zzz rows to compare with full table scans.

This is faster but with indexes could perform even better.

update CE_WORKSET_READ_ENTRY workset
set workSet.MARK = 0,
workSet.SELECTABLE = 0
where workSet.MARK = 1 and
exists (
select 1
from CE_WORKSET_READ_ENTRY source
where 
source.TRANSFORMED_TYPE = workSet.TRANSFORMED_TYPE
and source.TRANSFORMED_KEY =workSet.TRANSFORMED_KEY
and source.SELECTABLE = 0 
and source.MARK = 1);


If TRANSFORMED_TYPE and/or TRANSFORMED_KEY are unique then you can remove the subquery.