Why nios ii program execution time, sometimes fast, sometimes slow?

Thank you,

When xcpu into the disruption, we use the oscilloscope to see the external sram read signal line, there are signals, as if reading an external program, why read buffer will extend the program execution time? In addition, we found that interruption of proceedings, only a few programs , but the actual execution time of more than a theoretical program execution time, how to solve this problem? This is a nios own shortcomings, or do we design problem?