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After updating my MSYS2/Mingw-w64 environment to SDL 2.0.8, I noticed a performance loss of about 50% when audio synchronization is enabled compared to previous versions of the SDL2 library.

I compared it with my own compilation of the current master code with SDL 2.0.7 and a current nightly compiled by the buildbot. It doesn't matter if I try with a GB, GBC or GBA game as I always get the same results.

SDL 2.0.7 and 2.0.5:
Audio buffer size of 1024 samples: stable 60 FPS.
Audio buffer size of 786 samples: stable 60 FPS.
Audio buffer size of 512 samples: stable 60 FPS.
Audio buffer size of 384 samples: around 52 FPS.

SDL 2.0.8:
Audio buffer size of 1024 samples: stable 60 FPS.
Audio buffer size of 786 samples: around 48 FPS.
Audio buffer size of 512 samples: around 32.5 FPS.
Audio buffer size of 384 samples: around 23.5 FPS.

It is noteworthy that the performance drop clearly has something to do with the audio synchronization. If I untick audio sync in the settings, FPS for both versions get up into several thousand FPS.

OS: Windows 10, 1709, build 16299.309
Graphics hardware: AMD Radeon RX 560D
CPU: AMD Ryzen 5 1600
Onboard audio chip.
Latest drivers for everything.