In earlier versions, apps used a persistent cursor into the event stream and a short buffering window to track delivery and sort incoming events by timestamp. This implementation was based on the assumption that many apps would want loosely-ordered input, and providing this in the server rather than each app individually would be most efficient.
The flipside of this decision was that ordered delivery and a persistent cursor for each app still required quite substantial resources. Seq servers with many dashboard charts or running apps could spend a large amount of CPU and I/O time updating them. This turned out to be largely a waste, given the infrequent need for ordering, and the limited benefits of a persistent cursor when the event delivery process is inherently unreliable.
Seq 2.3 gains some big performance wins by using in-memory delivery of incoming events to apps. The overhead of running apps in this mode is negligible, so the difference can be noticeable. A loaded test server running just five charts used close to 70% less CPU time on 2.3 than on 2.2.
Because most apps benefit from the new model, this is the default on 2.3 servers. Apps that might behave differently with this change can opt back into ordered delivery using a new setting Order events by timestamp.
Hopefully the new version will make your server smile! :-)