If you still have the original hard drive, then that's the likeliest cause of slowness, and it should be replaced by an SSD - to improve reliability, if nothing else. (Indeed, one of the reasons they cut off older Macs is because the new OS doesn't run well.) If Apple says an OS will run on that Mac, that means it should run fine. Your question is based on the premise that it is the OS that is causing the slowness.