The builtin commands for SE is simple enough. But incoming events could need some more planning.
SE will have a set of events that is fired based on stuff that happens in OpenSim (EventHandler). This EventHandler will translate the event into a LSL-compatible event, including picking up the required parameters.
The EventHandler needs to know what object, and possibly what script, the event is for. Then it will queue this event for the correct script(s) (multiple scripts may subscribe to the same event in an object).
A Event Queue Manager will process the event queue under a separate thread and execute the correct script functions with the correct parameters. By using a separate thread OpenSims event thread is free to return immediately.
What scripts subscribe to what events is determined during script load. Either by dynamically identifying function names, or by having the script hook itself up during load. The functions will be stored by reference to avoid using expensive Reflection-calls for every script event.
One possiblity for extra speed is to dynamically (using CIL) create a call object in same AppDomain as script is located that does some of Event Queue Managers work.