I was thinking about adding a delay when I wrote this, but you loose the nice properties we currently have on portals being able to execute messages from L2 quickly with an optimistic style delay. I think this is dealbreaker IMO as it makes the UX way worse.
The one benefit of the delay approach though is it would be able to operate at a higher throughput. With the current validating nodes, you will be capped at ETH L1 style throughput while the training wheels are on as everyone has to execute everything…