If the SD card seems sluggish, you can try purchasing the "extreme" edition of the sdSD cards that are rated for professional photographers that need quick writes for RAW photos in fast succession. CostsIt costs more, but you get better performance.
AlternativeThe alternative is to wait for Dropbox to release a version that will have placeholders for files stored on the cloud only. The started in May 2016 and hopeI am hoping it will be released soon. This doesn't increase your overall storage, but does allow you to have Dropbox on the SSD and temporarily work with files and release them once you don't need them without having aan SD card.