The JIT acceleration does not gain full speed in debug mode. With the JIT compilation the order of executions can be influenced, such that line-by-line debugging would lead to surprising results.
Therefore a breakpoint in the code slows down the execution measureable, 10% in some tests. The effects of "dbstop if error" are smaller, about 1 to 2% in my tests. But this is very near to the measurement accuracy. So I assume the benefits for debugging will top te drawback for the speed.