Performance of `$table->groupsOnly()`

I want to create a reporting table of db-table with ~600k rows. For this, I added

  ->defaultGroup("ad_advertorial_id")
  ->groupsOnly()

  ->defaultGroup("ad_advertorial_id")
  ->groupsOnly()

to my table. First I got memory_limit errors, after increasing it to 1GB I get timeouts > 60s.

Any ideas?