BehaviorLibrary is a framework for creating behavior trees for game AI. It is free to use, modify, and redestribute as covered under the attached License (FreeBSD).
It is simple to use and with that simplicity comes performance.
Example of a simple A* following AI on a tilemap
//setup all coniditionals and their delegate functions
Coniditional tooClose = new Conditional(isTooClose);
Coniditional targetMoved = new Conditional(hasTargetMoved);
Coniditional pathFound = new Conditional(hasPathBeenFound);
Coniditional reachedCell = new Conditional(hasReachedCell);
Coniditional reachedTarget = new Conditional(hasReachedTarget);
Coniditional isNewPath = new Conditional(hasNewPath);
//setup all actions and their delegate functions
BehaviorAction moveToCell = new BehaviorAction(moveTowardsCell);
BehaviorAction calcPath = new BehaviorAction(calculatePath);
BehaviorAction initPathfinder = new BehaviorAction(initializePathfinder);
BehaviorAction getNextCell = new BehaviorAction(getNextPathCell);
BehaviorAction setPath = new BehaviorAction(setNewPath);
BehaviorAction getPath = new BehaviorAction(getCurrentPath);
BehaviorAction updatePosition = new BehaviorAction(updateTargetPosision);
BehaviorAction reset = new BehaviorAction(resetPathfinder);
BehaviorAction animate = new BehaviorAction(updateAnimation);
//setup an initilization branch
ParallelSequence initialize = new ParallelSequence(initPathfinder, calcPath);
//if the target has moved, reset and calculate a new path
ParallelSelector ifMovedCreateNewPath = new ParallelSelector(new Inverter(targetMoved), new Inverter(reset), calcPath);
ParallelSelector ifPathFoundGetPath = new ParallelSelector(new Inverter(pathFound), getPath);
ParallelSelector ifPathNewUseIt = new ParallelSelector(new Inverter(isNewPath), setPath);
ParallelSelector ifReachedCellGetNext = new ParallelSelector(new Inverter(reachedCell), getNextCell);
ParallelSelector ifNotReachedTargetMoveTowardsCell = new ParallelSelector(reachedTarget, moveToCell);
//follow target so long as you're not too close and then animate
ParallelSequence follow = new ParallelSequence(new Inverter(tooClose), updatePosition, ifMovedCreateNewPath, ifPathFoundGetPath, ifPathIsNewUseIt, ifReachedCellGetNext, ifNotReachedTargetMoveTowardsCell, animate);
//setup root node, choose initialization phase or pathing/movement phase
RootSelector root = new RootSelector(switchBehaviors, initialize, follow);
//set a reference to the root
Behavior behavior = new Behavior(root);
//to execute the behavior
behavior.Behave();