Must be something fishy with your dads machine, at least if Safari is slow. (sidenote: tried MS Office on my iMac about 6 months ago...it was a PAIN even on iMac - something really wrong in that program code). Safari on mine is almost as fast as the iMac, well at least I can't really see any major difference. I have used FCPX on my 2009 MBP, it worked ok - not superfast of course and rendering was slow, but for casual editing it was ok. Prior to buying the iMac the old MBP and FCPX was my main video editing machine. I removed it when I bought the iMac, mainly because of the screen size, not so much of performance. But sure, performance i blazingly fast on the iMac regarding movie editing. (Having both FCPX and iMovie, I plan to go back to iMovie because I do not need all the features that FCPX have, and iMovie work across iOS/OSX).
If you do a lot of movies I wouldn't go only MB or MBP, but rather look to iMac anyway. I regard the Macbook 12" more as a tool during traveling. And as such I recon it would be way better than my MBP 2009, and as earlier stated approx same Geekbench score as 2014/15 MBPr 13", which if true would be more than enough for most purposes.
I would say that the MB 12" would be close to perfect for photogs on tour using Lightroom, and work ok to casual video editing.