There was already a bug report - https://bugs.dolphin-emu.org/issues/13416. Furthermore, the Libretro port is not maintained by the Dolphin team, so your report will probably be closed/forwarded to the libretro repository.

So I finally bought Double Dash because I wanted to try it out on my Pi4 using dolphin-rpi. At first the game speed was really poor (~60% fullspeed). That said, I found this hack that gets Double Dash playing at fullspeed by cutting the framerate from 60fps to 30 and activating the game's internal frameskipping:

I added the following commands to the GM4.ini file in ~/DolphinConfig5.0/GameSettings:

[Core] EnableCheats = True [ActionReplay_Enabled] $Force 30FPS [ActionReplay] $Force 30FPS 0448C388 00000002

(source: https://www.reddit.com/r/gpdxd/comments/flq57f/near_fullspeed_double_dash_with_30fps_code/)

Note that my Pi4 is overclocked 2350MHz/940MHz. The games runs a bit choppy at around 23-28fps but the game speed seems to basically be fullspeed. It's very playable on the Pi4 with this hack.

I am not sure how helpful this hack will be for Pi5 users at is sounds like Double Dash runs at fullspeed regardless and I am not sure if this hack will help with multiplayer. That said, I mainly wanted to share this for any Pi4 users out there. It definitely made a huge difference for me playing this game.