[ArtTimeInvestor]

I don't see anything that looks like a "slowing" in mid 2018.

Growth in Q1, Q2 and Q3 2018 was higher than anything that came before.

[addaon]

Slowing refers to a change in the derivative, in this context. Slowing growth would be a decrease in change in GDP per year — a decrease in growth. But the claim is that the growth (first derivative) is what’s slowing — that is, the second derivative of GDP w.r.t. time went negative, which does seem to be the case in mid 2018 from the linked chart.