OS X handles that task. If Excel is becoming unresponsive there is another problem. How much total ram do you have? Which version of OS X are you running and which version of Excel?
Here's a page about how to use activity monitor to monitor memory usage
It could, also, be RAM memory related.
Your iMac model can take, at least, up to 16 GBs of RAM.
You could install, at least, another 4 GBs to bring your total RAM to 8 GBs.
You can purchase correct and reliable Mac RAM from online Mac RAM sellers Crucial memory or OWC (macsales).
I think you'll see a pretty significant change in performance of your iMac.
What you have to realize is Intel versions of OS X use up more and more CPU, GPU, RAM and hard drive resources.
OS X, by itself, needs between 2-4 GBs of RAM, for itself, to run smoothly, quickly and efficiently.
You add other applications running alongside OS X and the available RAM can be eaten up quickly by both the OS and the other apps you are running.
4 GBs of RAM is really a bare working minimum of RAM to allow your Mac to,at least, run decent out of the box.
Apple doesn't just give you free, extra RAM to allow for best or optimal performance.
You have to purchase and install that necessary RAM, yourself.
Thanks for your answer, what I do not understand is why there is still 2gb of free memory, while the CPU usage is 100% when the excel application goes in no response mode.
I have 2 Imacs, both have OSX 10.7.5, the one where the problem is severe has
4GB 1067 MHz RAM and a 3.06 GHz Intel Core 2 CPU
The other 4 GB 1333 MHz DDPR3 RAM and 2.7 GHz Intel Core Duo i5
The second works a lot better with the same files and size of data. That's why I thought it might be the CPU differenz. However, that would mean buying a new Imac.
Do you think adding RAM to the first above would solve the problem as well. Obviously cheaper as I think adding ram is possible to an older model
I would add ram to BOTH macs, 4GB just isn't enough. Also I assume you're using Activity Monitor, instead of looking at free and used ram look at page ins and page outs, that's a much better indicator of ram needs. http://support.apple.com/kb/ht1342
Finally I would look closely at the slow Mac, there's likely a problem there, regardless of how big your spread sheet is, it shouldn't be noticeably slower than the other, it's still just a spread sheet. How much total and free space is on the drive? What does Disk Utility report under SMART status?
As I implied, part of your issue is with the CPU and partly because of RAM.
My 2009 iMac has the same 3.06 Ghz Core2Duo Intel CPU as yours. The Core2Duo and i3 Intel CPUs are only dual core CPUs. Meaning that these CPUs process data through two data streams.
Your newer iMac while having a slightly slower CPU speed is, most likely, faster as it is a i5 quad core Intel processor, processing data through 4 data streams.
So, that is the difference.
RAM memory in both iMacs is still a significant factor.