Subsonic can be a bit memory intensive depending on size of library and streaming tasks; if not enough memory is allocated to it, performance really suffers. Your NAS model advertises only 512MB; that might be insufficient but as I don't believe there are posted memory requirements
https://github.com/ghenry22/Subsonic_Synology and I haven't tried on a unit with less than 8GB RAM, I couldn't say. You might use the Resource Monitor to look to see how much it is using during idle and active times.