Since we don't know what Mac you are using, it is difficult to understand what might be causing the slowdown while you are editing. I have been using iMovie with standard def and iMovie HD with high since they were available. I have used FinalCut for the past year. I have never noticed a "slowdown" during editing. I used to use a Canon Elura 60 and now use a Sony HDR-HC1. I also don't know what your threshold of pain is for "slow down". Please keep in mind that High Definition video is 4x more data to process - but it does NOT take 4x more time to render/process...
I used to use a G4 tower; I currently use a iMac flatpanel G5 and I have also used my son's iMac flatpanel CoreDuo. In my experience:
Importing HD video to either FinalCut or iMovie, it does not come across in realtime. Never tied it on the G4. On the G5, it is normally around 1/2 speed; on the CoreDuo, it is normally 3/4-full speed. When actually reviewing imported HD video, it plays at realtime. When doing transitions or adding credits (or other text) or somehow rendering on the computer, it depends what it is you are doing - the longer the "activity", the longer the render will take. But it does not seem any longer than when the G4 was doing the same on standard definition video.
Outputting or rendering to iDVD or DVD Studio seems to take less time on the two new machines than standard def took to render out standard def video on the G4... in any case, if it is a full single layer or double layer DVD, I click burn before I go to sleep at night and in the morning, the disc is done.
FinalCut *can* take less time during the import because it imports differently than iMovie does. You will use more hard drive space with FinalCut because you never really remove anything during editing/cutting from the imported file like you do with iMovie when you empty the iMovie trash...They call it "non-destructive editing"...
So... to answer your questions: HiDef video will "slow" the process because it will not import in realtime. We don't know how much you import - then cut - to result in your 6-8 minute film. FinalCut and iMovieHD will edit in realtime - though transitions, effects and titles will take a tad longer to render (than when you were in standard def) and DVD rendering will take a bit longer (than in standard def) because the HiDef video is being downsampled to standard def (so the is a LOT of crunching). But for 6-8 minutes final output, it should not take much more than 20 minutes to render on a G5 (less on a CoreDuo - I would not try it on a G4) - I do that pretty regularly for bands I've shot and edit their 3.5 minute to 5 minute songs rendered out to QuickTime clips for myspace or youtube upload.
If you are fortunate to have a CoreDuo MacPro tower, I would venture that the hidef import process would be realtime and the titles/credits/transitions will be *really* speedy... and the DVD render would be nearly instantaneous... hence no rendering time increase anywhere. Disc burn time is probably the same time over all the machines so not worth any discussion.
I hope this helps.