If the hash power tanks significantly, maybe that figure is around 50%, then yes there will be at least moderately rough period ( for the network), owing the the fact the blocks are basically full, until one or two difficulty adjustment kick in. At least a full 2 weeks.
The question is how many miners can you influence offline for what price point of bitcoin. The Chinese are predicting the halvening will only take 10-25% of hashing offline, but that is at a price of bitcoin between 600 to 700. But surely also some miners may mine through this period for the sake of the network, even if it is at loss.
the next difficulty adjustment is on the 18th of July. So the worst of the congestion if it materializes will last about a week, allowing a couple days for the hash power to reduce after the 9th.
RE: Possible scenario for MEGA BTC "flash" crash (manipulated by big miners)