Depends on the app. For many Nostr apps, the devs themselves sign the release and you get it just as fast as getting it directly from GitHub, with the added benefit of it being signed by their Nostr key.

However, most other apps are signed by nostr:npub10r8xl2njyepcw2zwv3a6dyufj4e4ajx86hz6v4ehu4gnpupxxp7stjt2p8, and there can be a delay. Usually not much of a delay, though.

I have generally moved to using the Zapstore, and only use Obtainium for apps that aren't available there. Which is surprisingly few.