Your formula contains CreateObject and/or CreateStatic object calls. While they are still supported, the performance of OLE in multi-threaded applications is poor, and you should consider replacing OLE with native AFL commands that do the same.
AB = CreateObject("Broker.Application"); //
OLE is slow
AmiBroker fully supports calling OLE objects from AFL formula level, and it is still safe to use, but there are technical reasons to advocate against using OLE. The foremost reason is that OLE is slow especially when called not from "owner" thread.
OLE was developed by Microsoft back in 1990's in the 16-bit days it is old technology and it effectivelly prevents threads from running at full speed as all OLE calls must be served by one and only user-interface thread. For more details see this article: http://blogs.msdn.com/b/oldnewthing/archive/2008/04/24/8420242.aspx
For this reason, if only possible you should strictly avoid using OLE / CreateObject in your formulas.
If you fail to do so, the performance will suffer. Any call to OLE from a worker thread causes posting a message to OLE hidden window and waiting for the main application UI thread to handle the request. If multiple threads do the same, the performance would easily degrade to single-thread level, because all OLE calls are handled by main UI thread anyway.
You should consider replacing OLE with native AFL commands that do the same.
For more details see Efficient use of multithreading