A typical BDM transaction between the debugger and the target goes like this. Debugger requests a memory-read from target. Debugger polls target for end of read.
Target indicates to debugger that the memory-read is complete.
Target passes memory-read results to debugger. The problem is that a memory-read on a bad address causes a bus fault error. So the polling routine must time-out before it can terminate. This time-out takes a long time, resulting in very slow window update speeds. The solution is to scroll the window so that bad memory is not visible.