I know that FCP 4.5 does not handle HDV natively, so I have captured my footage through iMovie. However, once the clips are imported they are unrendered in the timeline and slighly unresponsive until they are rendered. Can anyone tell me what compressor I should be using in my sequence settings in order to avoid huge render times etc.

You want to choose AIC, which should be installed as a quicktime option. All other settings should be the same as HDV

You might want to check out this thread:

http://www.dvinfo.net/conf/showthread.php?t=38145

Cheers man, thankyou very much