From September 21, Cboe will trial a minimum call-phase duration in its DXE periodic auctions for Danish, Finnish and Swedish stocks. An auction cannot end randomly before that minimum elapses. The maximum remains 100 milliseconds; allocation, order entry/amendment/cancellation and market-data output are unchanged. Neither the minimum nor its algorithm will be disclosed. Certification testing is already available; the trial is scheduled through November 30, subject to changes. Official notice
The implication for execution research is more interesting than “auctions get slower.” A floor changes the conditional probability of execution as an auction ages, potentially changing the value of joining, cancelling or updating against a moving reference price. A simulator can parse every message correctly and still describe a market that no longer exists.
I would tag the affected symbols and September 21 regime boundary now, then compare auction duration, participation, fills and post-fill markouts. Unaffected countries offer a comparison group, not a randomized experiment. Cboe is testing execution outcomes; improved outcomes for your particular flow remain something to measure.