Did you unchecked the "Emulate/Simulate AI" in the Audio Setting?
Old PC will experiences some slow down on Screen Menus/Messages from the game. They are minor slow down, they important thing is you can play with cheats.
If you don't planned to use Cheats, there is anothere Combo of Emu/Plugins/Settings for Old PC, in which you'll not experiences some slow down on Screen Menus/Messages from the game.
Code:
Best Combo To Play without Cheats:
Emulator:
Mupen64 v0.5.1
Video:
Rice's Video v5.9.9
Audio:
Azimer's HLE v0.56 WIP 2(you already have it)
RSP:
Azimer hle rsp plugin
APPLY THOSE SETTINGS:
Video Plugin configuration:
Frame Update at: At CI change
N64 Frame Buffer: Basic Framebuffer
Rendering to Texture: Hide Render-to-texture Effects
Comment:
Less graphics errors and a bit smoother.
Audio Plugin configuration:
Uncheck Emulate/Simulate AI
Comment:
Sound doesn't seem to stutter between cut scenes
or stages as much with this unchecked.
Good Luck & Have Fun!