PostgreSQL: How does the implicit index work when using "COPY" for loading bulk rows into a newly created table?
will the rows be indexed during "COPY" since the table has an implicit index?


Yes.


  if the answer is yes, the "COPY" will slow down?


Yes.

You can LOCK TABLE ... IN ACCESS EXCLUSIVE MODE then drop the PRIMARY KEY constraint for the duration if you need to.

PostgreSQL doesn't have a way to mark indexes as invalid then revalidate them for use without dropping and recreating them (yet).