It's a 350 microsecond delay to inbound orders meant to foil #HFT traders.
An economist at the SEC has studied its impact and found that it's good for investors:
wsj.com/articles/study…
"Intentional Access Delays, Market Quality, and Price Discovery: Evidence from IEX Becoming an Exchange" by SEC's Edwin Hu
papers.ssrn.com/sol3/papers.cf…
sec.gov/comments/s7-03…