Hi you dont need to write your own fastloader there is many of them out there that you can use , just don't forget to give the author credit for it.
For instance Krills loader https://csdb.dk/release/?id=226124 download and unpack and read the exampes and testfiles you can see how it works.There is 2 parts one part you run at init for the game and just once and then one other part you call when you want something to be loaded.
if you mean SD card oohickeys you mean SD2IEC its not optimal since its not emulateing the 1541 drive good at all, but 1541Ultimate series of cartriges is quite common(https://ultimate64.com/Main_products) though and it emulates a real 1541 drive , so you really want some kind of fastloader ,then can all who use booth the original 1541 drives series and with 1541ultimate loads it fast.