Slow-Moving Capital and Execution Costs: Evidence from a Major Trading Glitch

We investigate the impact of an exogenous trading glitch at a high-frequency market-making firm on standard measures of stock liquidity (effective and realized spreads) as well as on institutional trading costs (Implementation Shortfall and VWAP slippage) obtained from a proprietary data set. We find that stocks in which the firm accumulated large positions as a result of the trading glitch become substantially more illiquid on the day of the glitch. Effective spreads revert very quickly suggesting that market liquidity is resilient. Instead, institutional trading costs remain significantly higher for more than one week. We further document that all stocks for which the firm was a designated market maker become more illiquid, even if they were not heavily traded during the glitch, in the two days prior to being reassigned to another market maker. These findings are broadly consistent with 'slow-moving capital' theories and suggest that high-frequency trading 'flash crashes' may be associated with significant costs that are difficult to detect using standard liquidity measures.