Looking at your Hard Disk usage graph, you'll notice a huge spike of activity (to the top of the graph). Looking at your System Memory graph, I see lots of page-ins, and page-outs as well. And, I see Free memory is under 500 MB. As an unwritten rule, free memory should be
consistantly more than 500 MB (or its starving for more physical ram). I also see very few processes on the CPU. Thus, showing CPU is not being congested.
With these items in mind (without going into unversity level explanations and industry standard of less then 10% paging ratios), I'd say
your iMac system needs more physical memory. Since memory is very "inexpensive" these days, I would simply buy / install more memory. As much physical memory as your iMac model can take. Is that 4 GBs??? Not too sure. With more physical memory, your iMac would perform less "disk thrashing". re: Paging in/out because it's run out of memory. And, becuase the CPU must page out/in, some of its power is being used for disk activity - instead of processing the applicaton. Thus, negative impact on CPU, even though its not shown on its CPU chart.
Form more details about disk thrashing, surf:
http://www.webopedia.com/TERM/D/disk_thrashing.html
Long mumblings short... I would install more physical memory (4 GBs) and compare performance again. Since memory is inexpensive these days, I'd buy it now. Especially if you plan to keep your existing iMac.
Note: If I remember correctly, Lion recommends a minimum 2 GBs of memory. As more and more applications are added, the need for more memory increases. As an unwritten rule (based on my experience), I take the OS "minimum" recommendation and double it. In other words,
Lion really needs minimum 4 GBs of physical memory - for "acceptable" performance for the average home user.
Hope this helps in your research....
