This is common on underspecced computers that simply lack the hardware for the codec in use, but you forgot to give all those details, so this is only guessing.
Model Name: iMac
Model Identifier: iMac11,2
Processor Name: Intel Core i3
Processor Speed: 3.06 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache (per core): 256 KB
L3 Cache: 4 MB
Memory: 4 GB
Processor Interconnect Speed: 5.86 GT/s
Boot ROM Version: IM112.0057.B00
SMC Version (system): 1.64f5
Serial Number (system): W8045MJKDAS
Hardware UUID: CD8E4225-56B1-5836-B9C4-09E89CB4EC1A
If any of that helps at all^
Again guessing, you are using a more complex codec than SD DV.
SD DV on an iMAC with only an i3 dual core CPU, only 4 GB memory, a single hard disk and an AMD video card should run marginally acceptable, anything more complex is just not doable. Such an iMAC is at least 20 - 40 times slower than a good desktop and if you feed it too complex codecs, it is simply overwhelmed.