The internal storage medium of your router/TV box is unlikely to have any significant impact on your internet speed, assuming you could switch between HDD & SSD.
I read it as wanting to stream in standard definition SD rather than high def HD? I'm not familiar with that box do the apps e.g. Netflix have anything in their video/streaming settings
Control data usage: from a web browser go to your account page, then profile and parental controls. From a mobile device go to My Netflix and select Cellular Data Usage.