If we're talking dimensions, a low res texture pack may still be possible, im assuming the less colors that are used in a specific array of pixels, the lower the file size should be. Limit it to 3 colors and a transparency per sprite. Like an NES or SMS game. That should work shouldnt it?
Only things to improve performance are in the settings menus you can disable a bunch of visual details. Journey mode's ingame tweaks can impact performance. (I do NOT recommend speeding up time on low end computers)