Changes to the data in your sheet—changes to cells, adding or remove rows—are synced to apps within three minutes, although sometimes these changes will happen immediately.
Changes to the layout of your apps—changing components, themes, or other settings—can take up to 10 minutes to appear in existing apps, but should be available immediately for new users.
Changes to your app icon or splash screen often will not appear in existing apps unless the app is reinstalled. Android and iOS save your app's icon and splash screen when you add the app to your homescreen, and often will not refresh until the app is re-added.
Yes. Glide scales with your app, so there is no difference if a hundred, a thousand, or a million people open your app at the same time.