A warning for anyone using the EVENT_ACTION_SLOT_UPDATED event with the latest patch (2.3.5). When the player is wielding a Destruction Staff this event can be called 40-50 times (or more) whenever you kill something. I don't know how often it was called in previous patches but no where near this amount. Dual Wield and Restoration Staves have no issue with the event (it is not called on a mob kill at all). Bow and two-handed weapons have a similar issue but not all events on the bar are updated (only skill 4 in my test case, called 10 times per kill).

In my case it caused significant lag on kills as I was updating/saving the action bar with this event and assumed it would only be called when a skill on the bar was actually updated and then only once per skill within a small time frame. My fix was to check the time from the last call and only "process it" if X seconds have elapsed since the last call. This issue was not present in prior patches (or at least not to a visible degree).

For example, if I add simple logging to the EVENT_ACTION_SLOT_UPDATED event I get the following calls when I kill 1 mob with a Destruction Staff equipped:

EVENT_ACTION_SLOT_UPDATED 1
EVENT_ACTION_SLOT_UPDATED 1
EVENT_ACTION_SLOT_UPDATED 2
EVENT_ACTION_SLOT_UPDATED 4
EVENT_ACTION_SLOT_UPDATED 4
EVENT_ACTION_SLOT_UPDATED 4
EVENT_ACTION_SLOT_UPDATED 1
EVENT_ACTION_SLOT_UPDATED 2
EVENT_ACTION_SLOT_UPDATED 2
EVENT_ACTION_SLOT_UPDATED 1
EVENT_ACTION_SLOT_UPDATED 1
EVENT_ACTION_SLOT_UPDATED 2
EVENT_ACTION_SLOT_UPDATED 4
EVENT_ACTION_SLOT_UPDATED 4
EVENT_ACTION_SLOT_UPDATED 4
EVENT_ACTION_SLOT_UPDATED 1
EVENT_ACTION_SLOT_UPDATED 2
EVENT_ACTION_SLOT_UPDATED 2
EVENT_ACTION_SLOT_UPDATED 1
EVENT_ACTION_SLOT_UPDATED 1
EVENT_ACTION_SLOT_UPDATED 2
EVENT_ACTION_SLOT_UPDATED 4
EVENT_ACTION_SLOT_UPDATED 4
EVENT_ACTION_SLOT_UPDATED 4
EVENT_ACTION_SLOT_UPDATED 1
EVENT_ACTION_SLOT_UPDATED 2
EVENT_ACTION_SLOT_UPDATED 2
EVENT_ACTION_SLOT_UPDATED 1
EVENT_ACTION_SLOT_UPDATED 1
EVENT_ACTION_SLOT_UPDATED 2
EVENT_ACTION_SLOT_UPDATED 4
EVENT_ACTION_SLOT_UPDATED 4
EVENT_ACTION_SLOT_UPDATED 4
EVENT_ACTION_SLOT_UPDATED 1
EVENT_ACTION_SLOT_UPDATED 2
EVENT_ACTION_SLOT_UPDATED 2
EVENT_ACTION_SLOT_UPDATED 1
EVENT_ACTION_SLOT_UPDATED 1
EVENT_ACTION_SLOT_UPDATED 2
EVENT_ACTION_SLOT_UPDATED 4
EVENT_ACTION_SLOT_UPDATED 4
EVENT_ACTION_SLOT_UPDATED 4
EVENT_ACTION_SLOT_UPDATED 1
EVENT_ACTION_SLOT_UPDATED 2
EVENT_ACTION_SLOT_UPDATED 2

Which seems a little excessive. All these calls appear to happen simultaneously (within 3-4 ms).