I’ve been working with Mupen64Plus now for a week trying to get good performance because the latest Mupen core really thew me for a loop as it was completely unplayable. The thing I found to be my problem was vsync. As long as vsync was off I was able to get good performance at a constant 60 frames. As soon as I turned it on I got crackling audio and low frames. On a whim I ended up engaging vsync and triple buffering through the Nvidia control panel instead and turning it off in Retro Arch since that has helped me in the past with some games. That made all the difference in the world. Also make sure that hard GPU sync is on and that hard GPU sync frames is set atleast to 1 and not 0. I have my frame delay set to 5 but you may actually be able to set it higher. Games I’ve tested quite extensively so far are Perfect Dark, Pilotwings 64, Wave Race 64, Mario 64, and Rocket Robot on Wheels which all perform excellently.