Download speed difference between flashhd1 and flashhd2

[Nick Payne]

I notice a really marked difference in download speed between these two 
modes. I'm running get_iplayer 2.86 from the ppa 
jon-hedgerows/get-iplayer on Linux Mint 17, and I've noticed that if I 
specify --tvmode=best, then the download always first tries flashhd1 
mode, and that chugs along at a few hundred kilobit/sec. However, if I 
force flashhd2 mode with --tvmode=flashhd2, the download runs many times 
faster - up to 12-14 megabit/sec (the max line speed I get on a test to 
my ISP is around 16 megabit). Why this vast difference in the download 
speed of the two modes?

Nick