Movies are not in 1080p but 720p and that will suffice many. The streams come from the network of the Russian Facebook, vk.com, so it’s capacity should be alright, buffering will not happen much.
For subtitles, it is necessary to make an account on the site opensubtitles.org. This is free and can be done real quick.
When that’s done, you can log in and after choosing a film, you can set the subtitle in your preferred language. At the next movie or episode, it will show “None” again for subtitles, but if you click on that, it will search the subtitles and you can choose from a list. Which one you should choose is a matter of trying.
Quality and Server
You usually have a choice of 360p, 420p or 720p. If for example, 720p is missing, you can also choose another server to see if there is a 720p version.
Changing a server can indeed be useful if a stream is not going well.
By default, Showbox plays the video using the built VLC Media Player, but you can also choose the standard of your Android Player.