Normally when your first put in an SD card you have the option of formatting as internal storage. The disadvantage of this is it runs slower than your internal storage and there is risk if the SD card ever fails so you can lose what's on it. The advantage is if you have a phone that is out of internal memory an SD card can make the phone useable again and allows you to take the SD card to another phone or pc etc to retrieve files. I tend to use SD cards for media only but on occasions have used them on older phones that don't have enough storage for basic apps as a means of expanding the phones internal storage to allow app updates etc. if possible restrict the use of an SD card for media and non essential apps as some apps still use the phones internal storage even when an SD card is installed.
The best way of speeding up the phone is to uninstall unused apps and clear clutter such as cache files and surplus media etc and you can often utilise the free allowances in cloud storage options such as Google drive, Google photos Microsoft one drive etc for storing media and documents etc to save space on your phone. I hope this helps
