Your best bet is to keep an eye on the libretro/mgba GitHub.
Having said that, there is actually already an updated version of the core that has been out since 14 October (c758314). However, in the Loose Goose release notes, it seems like MuOS devs have reverted to an older (less performant) version of mGBA:
Reverted mGBA Libretro core to
affc86ebranch
I would recommend you try updating the core using the RetroArch online updated, and see if that resolves any issues. This issue is well-documented within this GitHub issue raised, and in the libretro/mGBA dev response within this pull request, which shows that the 14 October update was intended to resolve this slow-down issue.
This issue has been around for the last few Goose releases now, I posted a thread back in September and my solution in the end was to switch to the gpSP core.