Download an application called galaxy labs from the galaxy store, this app has some really great functions built in including a function called galaxy app booster.
Once galaxy apps is installed from the galaxy store open the app and allow all the persomissions, once done galaxy app booster will be one of the apps which will need to be downloaded also.
Once downloaded open this and run it, (you can run it in the background). The more apps you have the longer it will take but this will help improve on the phone being slow.
As for the amazon app I would suggest you go to the play store and search for the app and then click on the reviews and then click the 3 lines on the top right and select for this device only and have a look at the reviews. This most of the times will give you an indication of how many others are having the issue and will help identify if it is an app issue I personally believe it is an app issue.
