Have 20 in. mid-2007 iMac with 3GB RAM. I removed the 1GB and replaced with a 2GB maxing out the RAM for this model. The 4MB RAM shows up in the settings as "good", but the computer is running incredibly slow with the rainbow pinwheel showing up every time I try a new task. El Capitan was running fine before albeit a bit slow. Nothing else changed. Any thoughts?