Comment 2 for bug 425650

os-prober 1.31 will appear soon (not uploaded to Ubuntu yet - I'm waiting for it to land in Debian unstable first). When it does, could you please try with that and let me know if it's noticeably faster? It ought to be.

Regarding the duplicates, you can probably remove /etc/grub.d/30_otheros, which was mistakenly created by the installer (and won't be created in fresh installations as of Alpha 5).