I developed two separate Excel applications using VBA, each has about 10 class modules. A strange thing happened after I combined both applications under one file (i.e., all class modules and worksheets are in one file): the speed of one application, app A, would run as much as 5 times slower than before when the other application was not running (the two applications are always run separately and never simultaneously). If I delete just the worksheets associated with the other application, app B, then app A's performance would get back to normal. If I delete only the class modules associated with app B, then app A's performance would improve, but not by much. I don't understand why one application would affect the other's performance when the other application was not even running. Has anyone had this problem before? How can I diagnose the root cause of this performance issue?