One area where you'll find the AST debugger ��worse is in performance.�� The bytecode debugger uses perform: to execute at full speed when possible (it does interpret when doing step into or through, but not when doing over).�� In fact this part of the system, because it has to catch no local returns and exceptions, is the most complex, but it's not because of VM details, it's because of the use of perform: to run code being debugged to run at full speed.