Synology Photos Slow - eviltoast

I recently acquired a second hand DS216j with 6TB (SHR1 so only 3TB is usable). I moved all my Google Photos images and videos onto it, but now when I access it through QuickConnect it loads them really slow, the videos are constantly buffering and when they load they have a messed up framerate (really jittery). I have a 500 fibre connection and the station is connected to the router through an ethernet cable. Any idea how I could possibly fix this? Thanks