Emulator

Is this from the wiki or did you personally test this? Does it happen in deterministic GPU or syncGPU, or just uncapped dualcore?

From the wiki, but also personally tested.

On closer review this actually seems to be a problem with Dolphin requesting CPU time when there are other processes competing for it. Even low priority outside tasks seem to prevent Dolphin from using appropriate CPU cycles. This is apparent when running Distributed.net, SETI, or other similar apps that consume available CPU cycles.

By the way, I wanted to try this myself, and found the heavy fps drop is near identical to the fps drops in the devolution loader for Wii. When I noticed that problem in 2014 I thought it was strange that a game running on the Wii CPU speed would do something like that, but apparently it has to do with how devolution works. It happens on other games to a much lesser extent (Sonic DX), usually when there's a lot of models on screen. I recall tueidj simply stating that it only happens on poorly optimized games.