From the Docs
1964 1.1
If the video plugin supports this option, enabling RSP/RDP timing will allow the video plugin to provide to 1964 a better estimation of elapsed N64 cycles required to finish a display list. Then, 1964 will delay the respective interrupts accordingly. Rice's hope was that this options could give better timing for time-sensitive games, but we haven't seen this feature help any games. You can leave it off, and we may eventually remove it.