Emulator

Game Name?

Sonic Mega Collection

Game ID? (right click the game in the game list, Properties, Info tab)

GSOE8P

MD5 Hash? (right click the game in the game list, Properties, Verify tab, Verify Integrity button)

Verification is crashing Dolphin. This is a good dump though.

What's the problem? Describe what went wrong.

In Sonic 3 and Knuckles, there's a save select screen with a bunch of fancy effects. It turns out that the emulator lags in Dolphin but not on a real GameCube. That lag causes a very big mess of graphics and other issues. You can tell it's lagging in game because the framerate will read 40 FPS/60 VPS.

What steps will reproduce the problem?

Use the savefile provided
select Sonic 3 and Knuckles. Not Sonic 3, not Sonic and Knuckles

Is the issue present in the latest development version? For future reference, please also write down the version number of the latest development version.

This issue is present in all builds of Dolphin. It was missed previously due to the fact that it is not present in Sonic 3 and Sonic and Knuckles. You need a savefile to access Sonic 3 and Knuckles from what I can tell.

Is the issue present in the latest stable version?

Yep

What are your PC specifications? (CPU, GPU, Operating System, more)

Core i7-6700K, GTX 1070, Windows 10

Is there anything else that can help developers narrow down the issue? (e.g. logs, screenshots,
configuration files, savefiles, savestates)

Setting the emulated CPU clockrate to 109% or higher seems to remove the lag and fix the rendering of the screen. There seems to be additional lag in other areas at 100%, but they don't cause such an obvious malfunction.