Avoid random apps with numerous ads and weird permissions. Read reviews, see what they are asking for, and beware of apps that ask for your Instagram password. The best Instagram download app ought to just require the link to the post and access to storage. Boostbysocial has a very helpful comparison to help you determine what to look out for before installing anything. If an app is slow or continues to pop up, remove it immediately and use a browser based app, as this is generally safer.