pt-table-sync slow on first sync

If i have 2 servers, that the main server have around 400 000 rows in the table i want to sync, and at the other server its still empty.
Why dose it take hours to do this first sync?
Its alot faster to do a mysqldump of it, transfer that file, and import that file.

The conclusion of “I have 400 000 rows in my primary key at server 1, and 0 rows on server 2, I think i need to insert all rows in to server 2”,
sounds easy, so why is it taking over 10 times longer then a mysqldump/transfer/import?

Where can i Look for bottlenecks?