Actionbar | The actionbar utility class has been completely rewritten. Prior to this update NMS classes, constructors, methods, and fields were all accessed directly via reflection every time a message was sent to a player; that is quite resource-intensive if used frequently.
This update ensures that all of those code related entities are cached and reused. This means that they are only accessed during startup and then reused as needed when messages are requested. This will have a significant positive impact on performance.
Please note that this only concerns 1.8, and 1.9 servers. 1.10 and up use the built-in method in the Spigot API for sending these kinds of messages.