Mario Galaxy major slowdown when bright lights on screen

[napsack]

Running Mario Galaxy at 3x native, everything is running 60fps at 1080p with no hiccups until the bright lights in the background come on screen, at which point the FPS drops to ~25 and everything stutters to the point of almost being unplayable. As soon as the light is off screen, fps jump to 60 again. Throughout the game, there is a bright galaxy shaped light on the horizon always visible. No other effects or animations cause slowdown for me.

I've tried every gfx backend, all the same results. I even set Dolphin to 1x native with no enhancements, and it didn't solve the issue. I've changed every combo of settings I can think of and haven't been able to solve this one. Other games run just fine.

Screenshots (notice fps in corner)

Here are my settings:

System specs: 

CPU- Core i7 4770 
GPU- AMD RX 460 
Ram- 8gb 
OS- Windows 10 
Dolphin 5.0-1580

Any ideas?

[admin89]

You should try the latest dev Dolphin . I don't have any issue with this game
4xIR , FXAA 60FPS all the time , no lag on my laptop (i7 4700MQ @ 3.6GHz + GTX 860M)
Edit : Nevermind, I thought you were using the official Dolphin 5.0 .
Btw, I'm using the latest Ishiiruka . So it might be a little different from latest dev Dolphin but I max out every single option for the best quality as possible , still no slowdown