It won’t be nearly as fast, but you could easily have a slim fit storage device in the SD slot. You’d want some microsd to sd adapter made specifically for the MacBook.
I've done that, but there is a caveat. All SD cards are optimized for sequentional read/write (photos/videos) and are very slow for random access. In real life scenarios it can become very slow and sometimes the bandwidth wouldn't be enough for HD videos to load normally.
