The model iMac you have is not optimal for FCPX but it should be able to handle the majority of projects that are not overly complex without freezing. Check Activity Monitor to see what's going on while your project is open.
Do you have many events in a single library?
What format are the original clips and did you optimize?
How are your external drives connected?
Like Russ said, check Activity Monitor and see what else is running and turn off any Applications that aren't needed.
Turn off wi-fi or disconnect the wire that goes to the Internet
Turn off Background Rendering (if you see an orange line on the top of the time line that moves along and gets shorter while you're not doing anything that is a good sign that Background Rendering is running.
I haven't used 10.1 yest but on the previous version there was a small circle approximately in the middle of the screen, about 1/2-inch or 1 cm in diameter, and it would have a number inside. The number is the percent complete of what is hapening. When it is Rendering the percentage will be shown.
Basically, you probably need to free up resources.
For what it is worth, my previous Mac Pro was a 1,1 2.66 machine, also with 8GB of, I think, 667 memory, and it would process FCPX 10.0.9 okay. It did have an upgraded video card, though.