Wow, this is a useless editorial.
No link to the report, unclear if the report takes into account years since migration (it takes time to learn language, develop networks, and climb ladders), some indication that the trouble is that migrants end up in low paying jobs (which of course would decrease GDP), and no comment on the fairly obvious question on what the integration policy says about time frames.
Also, it puts all of the post-Brexit decline at the door of the immigrants, which seems ridiculous.
This reads like a hit piece from conservatives in preparation for election season.